diff --git a/samples/book-app-project/book_app.py b/samples/book-app-project/book_app.py
index f0100c2d..a6230f1f 100644
--- a/samples/book-app-project/book_app.py
+++ b/samples/book-app-project/book_app.py
@@ -1,65 +1,70 @@
 import sys
 from books import BookCollection
+from utils import print_books
 
 
 # Global collection instance
 collection = BookCollection()
 
 
-def show_books(books):
-    """Display books in a user-friendly format."""
-    if not books:
-        print("No books found.")
-        return
-
-    print("\nYour Book Collection:\n")
-
-    for index, book in enumerate(books, start=1):
-        status = "✓" if book.read else " "
-        print(f"{index}. [{status}] {book.title} by {book.author} ({book.year})")
-
-    print()
-
-
-def handle_list():
+def handle_list() -> None:
+    """Display all books in the collection."""
     books = collection.list_books()
-    show_books(books)
+    print_books(books)
 
 
-def handle_add():
+def handle_add() -> None:
+    """Prompt user to add a new book to the collection."""
     print("\nAdd a New Book\n")
 
     title = input("Title: ").strip()
     author = input("Author: ").strip()
     year_str = input("Year: ").strip()
 
+    if not title or not author:
+        print("\nError: Title and author cannot be empty.\n")
+        return
+
     try:
         year = int(year_str) if year_str else 0
         collection.add_book(title, author, year)
         print("\nBook added successfully.\n")
-    except ValueError as e:
-        print(f"\nError: {e}\n")
+    except ValueError:
+        print("\nError: Year must be a valid number.\n")
 
 
-def handle_remove():
+def handle_remove() -> None:
+    """Prompt user to remove a book from the collection by title."""
     print("\nRemove a Book\n")
 
     title = input("Enter the title of the book to remove: ").strip()
-    collection.remove_book(title)
 
-    print("\nBook removed if it existed.\n")
+    if not title:
+        print("\nError: Title cannot be empty.\n")
+        return
+
+    if collection.remove_book(title):
+        print("\nBook removed successfully.\n")
+    else:
+        print("\nBook not found.\n")
 
 
-def handle_find():
+def handle_find() -> None:
+    """Prompt user to find books by author."""
     print("\nFind Books by Author\n")
 
     author = input("Author name: ").strip()
-    books = collection.find_by_author(author)
 
-    show_books(books)
+    if not author:
+        print("\nError: Author name cannot be empty.\n")
+        return
 
+    books = collection.find_by_author(author)
+    print_books(books)
 
-def show_help():
+
+def show_help() -> None:
+    """Display help message with available commands."""
     print("""
 Book Collection Helper
 
@@ -72,23 +77,26 @@ def show_help():
 """)
 
 
-def main():
+def main() -> None:
+    """Parse command-line arguments and dispatch to appropriate handler."""
     if len(sys.argv) < 2:
         show_help()
         return
 
     command = sys.argv[1].lower()
 
-    if command == "list":
-        handle_list()
-    elif command == "add":
-        handle_add()
-    elif command == "remove":
-        handle_remove()
-    elif command == "find":
-        handle_find()
-    elif command == "help":
-        show_help()
+    commands: dict[str, callable] = {
+        "list": handle_list,
+        "add": handle_add,
+        "remove": handle_remove,
+        "find": handle_find,
+        "help": show_help,
+    }
+
+    handler = commands.get(command)
+
+    if handler:
+        handler()
     else:
         print("Unknown command.\n")
         show_help()
diff --git a/samples/book-app-project/tests/test_books.py b/samples/book-app-project/tests/test_books.py
index 061149c5..8a5796b3 100644
--- a/samples/book-app-project/tests/test_books.py
+++ b/samples/book-app-project/tests/test_books.py
@@ -15,39 +15,367 @@ def use_temp_data_file(tmp_path, monkeypatch):
     monkeypatch.setattr(books, "DATA_FILE", str(temp_file))
 
 
-def test_add_book():
-    collection = BookCollection()
-    initial_count = len(collection.books)
-    collection.add_book("1984", "George Orwell", 1949)
-    assert len(collection.books) == initial_count + 1
-    book = collection.find_book_by_title("1984")
-    assert book is not None
-    assert book.author == "George Orwell"
-    assert book.year == 1949
-    assert book.read is False
-
-def test_mark_book_as_read():
-    collection = BookCollection()
-    collection.add_book("Dune", "Frank Herbert", 1965)
-    result = collection.mark_as_read("Dune")
-    assert result is True
-    book = collection.find_book_by_title("Dune")
-    assert book.read is True
-
-def test_mark_book_as_read_invalid():
-    collection = BookCollection()
-    result = collection.mark_as_read("Nonexistent Book")
-    assert result is False
-
-def test_remove_book():
-    collection = BookCollection()
-    collection.add_book("The Hobbit", "J.R.R. Tolkien", 1937)
-    result = collection.remove_book("The Hobbit")
-    assert result is True
-    book = collection.find_book_by_title("The Hobbit")
-    assert book is None
-
-def test_remove_book_invalid():
-    collection = BookCollection()
-    result = collection.remove_book("Nonexistent Book")
-    assert result is False
+class TestAddBook:
+    """Tests for adding books to the collection."""
+
+    def test_add_book_success(self):
+        """Test adding a valid book."""
+        collection = BookCollection()
+        book = collection.add_book("1984", "George Orwell", 1949)
+        
+        assert len(collection.books) == 1
+        assert book.title == "1984"
+        assert book.author == "George Orwell"
+        assert book.year == 1949
+        assert book.read is False
+
+    def test_add_book_persists_to_file(self):
+        """Test that added books are saved to data.json."""
+        collection = BookCollection()
+        collection.add_book("Dune", "Frank Herbert", 1965)
+        
+        collection2 = BookCollection()
+        book = collection2.find_book_by_title("Dune")
+        assert book is not None
+        assert book.author == "Frank Herbert"
+
+    def test_add_multiple_books(self):
+        """Test adding multiple books."""
+        collection = BookCollection()
+        collection.add_book("Book 1", "Author A", 2000)
+        collection.add_book("Book 2", "Author B", 2001)
+        collection.add_book("Book 3", "Author C", 2002)
+        
+        assert len(collection.books) == 3
+
+    def test_add_book_with_zero_year(self):
+        """Test adding a book with year 0 (default)."""
+        collection = BookCollection()
+        book = collection.add_book("Anonymous", "Unknown", 0)
+        
+        assert book.year == 0
+
+    def test_add_book_with_duplicate_title(self):
+        """Test that duplicate titles are allowed."""
+        collection = BookCollection()
+        collection.add_book("Title", "Author A", 2000)
+        collection.add_book("Title", "Author B", 2001)
+        
+        assert len(collection.books) == 2
+
+
+class TestRemoveBook:
+    """Tests for removing books from the collection."""
+
+    def test_remove_book_success(self):
+        """Test removing an existing book."""
+        collection = BookCollection()
+        collection.add_book("The Hobbit", "J.R.R. Tolkien", 1937)
+        
+        result = collection.remove_book("The Hobbit")
+        
+        assert result is True
+        assert len(collection.books) == 0
+        assert collection.find_book_by_title("The Hobbit") is None
+
+    def test_remove_book_case_insensitive(self):
+        """Test that removal is case-insensitive."""
+        collection = BookCollection()
+        collection.add_book("The Hobbit", "J.R.R. Tolkien", 1937)
+        
+        result = collection.remove_book("the hobbit")
+        
+        assert result is True
+        assert collection.find_book_by_title("The Hobbit") is None
+
+    def test_remove_book_not_found(self):
+        """Test removing a non-existent book."""
+        collection = BookCollection()
+        
+        result = collection.remove_book("Nonexistent Book")
+        
+        assert result is False
+        assert len(collection.books) == 0
+
+    def test_remove_book_from_empty_collection(self):
+        """Test removing from empty collection."""
+        collection = BookCollection()
+        
+        result = collection.remove_book("Any Book")
+        
+        assert result is False
+
+    def test_remove_book_persists_to_file(self):
+        """Test that removal is persisted to data.json."""
+        collection = BookCollection()
+        collection.add_book("Book to Remove", "Author", 2000)
+        collection.remove_book("Book to Remove")
+        
+        collection2 = BookCollection()
+        assert collection2.find_book_by_title("Book to Remove") is None
+
+    @pytest.mark.parametrize("title", ["", " ", None])
+    def test_remove_book_with_empty_input(self, title):
+        """Test removing with empty or None input."""
+        collection = BookCollection()
+        collection.add_book("Valid Book", "Author", 2000)
+        
+        result = collection.remove_book(title) if title is not None else collection.remove_book("")
+        
+        assert result is False
+        assert len(collection.books) == 1
+
+
+class TestFindByTitle:
+    """Tests for finding books by title."""
+
+    def test_find_book_by_title_success(self):
+        """Test finding an existing book by title."""
+        collection = BookCollection()
+        collection.add_book("1984", "George Orwell", 1949)
+        
+        book = collection.find_book_by_title("1984")
+        
+        assert book is not None
+        assert book.author == "George Orwell"
+        assert book.year == 1949
+
+    def test_find_book_by_title_case_insensitive(self):
+        """Test that title search is case-insensitive."""
+        collection = BookCollection()
+        collection.add_book("Dune", "Frank Herbert", 1965)
+        
+        book = collection.find_book_by_title("dune")
+        
+        assert book is not None
+        assert book.author == "Frank Herbert"
+
+    def test_find_book_by_title_not_found(self):
+        """Test finding a non-existent book."""
+        collection = BookCollection()
+        collection.add_book("Book A", "Author A", 2000)
+        
+        book = collection.find_book_by_title("Book B")
+        
+        assert book is None
+
+    def test_find_book_by_title_from_empty_collection(self):
+        """Test finding a book in empty collection."""
+        collection = BookCollection()
+        
+        book = collection.find_book_by_title("Any Book")
+        
+        assert book is None
+
+    @pytest.mark.parametrize("title", ["", " "])
+    def test_find_book_by_empty_title(self, title):
+        """Test finding with empty title."""
+        collection = BookCollection()
+        collection.add_book("Valid Book", "Author", 2000)
+        
+        book = collection.find_book_by_title(title)
+        
+        assert book is None
+
+    def test_find_book_returns_first_match_with_duplicates(self):
+        """Test behavior when multiple books have same title."""
+        collection = BookCollection()
+        collection.add_book("Title", "Author A", 2000)
+        collection.add_book("Title", "Author B", 2001)
+        
+        book = collection.find_book_by_title("Title")
+        
+        assert book is not None
+        assert book.author == "Author A"
+
+
+class TestFindByAuthor:
+    """Tests for finding books by author."""
+
+    def test_find_by_author_single_book(self):
+        """Test finding books by author with single match."""
+        collection = BookCollection()
+        collection.add_book("Book", "Stephen King", 1990)
+        
+        books = collection.find_by_author("Stephen King")
+        
+        assert len(books) == 1
+        assert books[0].title == "Book"
+
+    def test_find_by_author_multiple_books(self):
+        """Test finding multiple books by same author."""
+        collection = BookCollection()
+        collection.add_book("Carrie", "Stephen King", 1974)
+        collection.add_book("The Shining", "Stephen King", 1977)
+        collection.add_book("It", "Stephen King", 1986)
+        
+        books = collection.find_by_author("Stephen King")
+        
+        assert len(books) == 3
+        assert all(book.author == "Stephen King" for book in books)
+
+    def test_find_by_author_case_insensitive(self):
+        """Test that author search is case-insensitive."""
+        collection = BookCollection()
+        collection.add_book("Dune", "Frank Herbert", 1965)
+        
+        books = collection.find_by_author("frank herbert")
+        
+        assert len(books) == 1
+        assert books[0].author == "Frank Herbert"
+
+    def test_find_by_author_not_found(self):
+        """Test finding books by non-existent author."""
+        collection = BookCollection()
+        collection.add_book("Book", "Author A", 2000)
+        
+        books = collection.find_by_author("Author B")
+        
+        assert books == []
+
+    def test_find_by_author_empty_collection(self):
+        """Test finding books in empty collection."""
+        collection = BookCollection()
+        
+        books = collection.find_by_author("Any Author")
+        
+        assert books == []
+
+    @pytest.mark.parametrize("author", ["", " "])
+    def test_find_by_author_empty_input(self, author):
+        """Test finding with empty author name."""
+        collection = BookCollection()
+        collection.add_book("Book", "Author", 2000)
+        
+        books = collection.find_by_author(author)
+        
+        assert books == []
+
+    def test_find_by_author_preserves_order(self):
+        """Test that results are in collection order."""
+        collection = BookCollection()
+        collection.add_book("Book C", "Author", 2002)
+        collection.add_book("Book A", "Author", 2000)
+        collection.add_book("Book B", "Author", 2001)
+        
+        books = collection.find_by_author("Author")
+        
+        assert [b.title for b in books] == ["Book C", "Book A", "Book B"]
+
+
+class TestMarkAsRead:
+    """Tests for marking books as read."""
+
+    def test_mark_as_read_success(self):
+        """Test marking an existing book as read."""
+        collection = BookCollection()
+        collection.add_book("Dune", "Frank Herbert", 1965)
+        
+        result = collection.mark_as_read("Dune")
+        
+        assert result is True
+        book = collection.find_book_by_title("Dune")
+        assert book.read is True
+
+    def test_mark_as_read_persists(self):
+        """Test that read status is persisted to data.json."""
+        collection = BookCollection()
+        collection.add_book("Dune", "Frank Herbert", 1965)
+        collection.mark_as_read("Dune")
+        
+        collection2 = BookCollection()
+        book = collection2.find_book_by_title("Dune")
+        assert book.read is True
+
+    def test_mark_as_read_case_insensitive(self):
+        """Test that marking as read is case-insensitive."""
+        collection = BookCollection()
+        collection.add_book("Dune", "Frank Herbert", 1965)
+        
+        result = collection.mark_as_read("dune")
+        
+        assert result is True
+        assert collection.find_book_by_title("Dune").read is True
+
+    def test_mark_as_read_invalid_book(self):
+        """Test marking non-existent book as read."""
+        collection = BookCollection()
+        
+        result = collection.mark_as_read("Nonexistent Book")
+        
+        assert result is False
+
+    def test_mark_as_read_empty_collection(self):
+        """Test marking book as read in empty collection."""
+        collection = BookCollection()
+        
+        result = collection.mark_as_read("Any Book")
+        
+        assert result is False
+
+    def test_mark_already_read_book(self):
+        """Test marking already-read book as read again."""
+        collection = BookCollection()
+        collection.add_book("Book", "Author", 2000)
+        collection.mark_as_read("Book")
+        
+        result = collection.mark_as_read("Book")
+        
+        assert result is True
+        assert collection.find_book_by_title("Book").read is True
+
+    def test_mark_multiple_books_as_read(self):
+        """Test marking multiple different books as read."""
+        collection = BookCollection()
+        collection.add_book("Book 1", "Author", 2000)
+        collection.add_book("Book 2", "Author", 2001)
+        
+        collection.mark_as_read("Book 1")
+        collection.mark_as_read("Book 2")
+        
+        assert collection.find_book_by_title("Book 1").read is True
+        assert collection.find_book_by_title("Book 2").read is True
+
+
+class TestEmptyCollection:
+    """Tests for edge cases with empty collection."""
+
+    def test_list_books_empty_collection(self):
+        """Test listing books in empty collection."""
+        collection = BookCollection()
+        
+        books = collection.list_books()
+        
+        assert books == []
+
+    def test_find_by_title_empty_collection(self):
+        """Test finding book in empty collection."""
+        collection = BookCollection()
+        
+        book = collection.find_book_by_title("Any Book")
+        
+        assert book is None
+
+    def test_find_by_author_empty_collection(self):
+        """Test finding books by author in empty collection."""
+        collection = BookCollection()
+        
+        books = collection.find_by_author("Any Author")
+        
+        assert books == []
+
+    def test_remove_from_empty_collection(self):
+        """Test removing book from empty collection."""
+        collection = BookCollection()
+        
+        result = collection.remove_book("Any Book")
+        
+        assert result is False
+
+    def test_mark_as_read_empty_collection(self):
+        """Test marking book as read in empty collection."""
+        collection = BookCollection()
+        
+        result = collection.mark_as_read("Any Book")
+        
+        assert result is False
diff --git a/samples/book-app-project/tests/test_utils.py b/samples/book-app-project/tests/test_utils.py
new file mode 100644
index 00000000..08743786
--- /dev/null
+++ b/samples/book-app-project/tests/test_utils.py
@@ -0,0 +1,14 @@
+from books import Book
+from utils import print_books
+
+
+def test_print_books_with_no_books(capsys):
+    print_books([])
+
+    assert capsys.readouterr().out == "No books found.\n"
+
+
+def test_print_books_displays_book_status(capsys):
+    print_books([Book("Dune", "Frank Herbert", 1965, read=True)])
+
+    assert "1. [x] Dune by Frank Herbert (1965)" in capsys.readouterr().out
diff --git a/samples/book-app-project/utils.py b/samples/book-app-project/utils.py
index 4151dcda..72227810 100644
--- a/samples/book-app-project/utils.py
+++ b/samples/book-app-project/utils.py
@@ -1,3 +1,6 @@
+from books import Book
+
+
 def print_menu():
     print("\n📚 Book Collection App")
     print("1. Add a book")
@@ -25,12 +28,16 @@ def get_book_details():
     return title, author, year
 
 
-def print_books(books):
+def print_books(books: list[Book]) -> None:
+    """Display books in a user-friendly format."""
     if not books:
-        print("No books in your collection.")
+        print("No books found.")
         return
 
-    print("\nYour Books:")
+    print("\nYour Book Collection:\n")
+
     for index, book in enumerate(books, start=1):
-        status = "✅ Read" if book.read else "📖 Unread"
-        print(f"{index}. {book.title} by {book.author} ({book.year}) - {status}")
+        status = "x" if book.read else " "
+        print(f"{index}. [{status}] {book.title} by {book.author} ({book.year})")
+
+    print()