diff --git a/samples/book-app-project-cs/Program.cs b/samples/book-app-project-cs/Program.cs
index 54373423..c9218774 100644
--- a/samples/book-app-project-cs/Program.cs
+++ b/samples/book-app-project-cs/Program.cs
@@ -59,9 +59,12 @@ void HandleRemove()
 
     Console.Write("Enter the title of the book to remove: ");
     var title = Console.ReadLine()?.Trim() ?? "";
-    collection.RemoveBook(title);
+    var removed = collection.RemoveBook(title);
 
-    Console.WriteLine("\nBook removed if it existed.\n");
+    if (removed)
+        Console.WriteLine($"\n\"{title}\" has been removed.\n");
+    else
+        Console.WriteLine($"\nNo book with title \"{title}\" was found.\n");
 }
 
 void HandleFind()
diff --git a/samples/book-app-project-cs/Services/BookCollection.cs b/samples/book-app-project-cs/Services/BookCollection.cs
index b219ba53..87ddef03 100644
--- a/samples/book-app-project-cs/Services/BookCollection.cs
+++ b/samples/book-app-project-cs/Services/BookCollection.cs
@@ -54,7 +54,7 @@ public Book AddBook(string title, string author, int year)
         return book;
     }
 
-    public List<Book> ListBooks() => _books;
+    public List<Book> ListBooks() => _books.ToList();
 
     public Book? FindBookByTitle(string title)
     {
diff --git a/samples/book-app-project-cs/Tests/BookCollectionTests.cs b/samples/book-app-project-cs/Tests/BookCollectionTests.cs
index 5a0aa829..b9db43b8 100644
--- a/samples/book-app-project-cs/Tests/BookCollectionTests.cs
+++ b/samples/book-app-project-cs/Tests/BookCollectionTests.cs
@@ -67,4 +67,102 @@ public void RemoveBook_NonexistentBook_ShouldReturnFalse()
         var result = _collection.RemoveBook("Nonexistent Book");
         Assert.False(result);
     }
+
+    [Fact]
+    public void ListBooks_ShouldReturnCopy_AndPreventExternalMutation()
+    {
+        _collection.AddBook("Dune", "Frank Herbert", 1965);
+        var books = _collection.ListBooks();
+        books.Clear();
+
+        Assert.Single(_collection.Books);
+    }
+
+    // --- FindByAuthor ---
+
+    [Fact]
+    public void FindByAuthor_ExactMatch_ShouldReturnBook()
+    {
+        _collection.AddBook("Dune", "Frank Herbert", 1965);
+        var results = _collection.FindByAuthor("Frank Herbert");
+
+        Assert.Single(results);
+        Assert.Equal("Dune", results[0].Title);
+    }
+
+    [Fact]
+    public void FindByAuthor_CaseInsensitive_ShouldReturnBook()
+    {
+        _collection.AddBook("Dune", "Frank Herbert", 1965);
+        var results = _collection.FindByAuthor("frank herbert");
+
+        Assert.Single(results);
+        Assert.Equal("Dune", results[0].Title);
+    }
+
+    [Fact]
+    public void FindByAuthor_MultipleBooksSameAuthor_ShouldReturnAll()
+    {
+        _collection.AddBook("A Wizard of Earthsea", "Ursula K. Le Guin", 1968);
+        _collection.AddBook("The Tombs of Atuan", "Ursula K. Le Guin", 1971);
+        var results = _collection.FindByAuthor("Ursula K. Le Guin");
+
+        Assert.Equal(2, results.Count);
+        Assert.Contains(results, b => b.Title == "A Wizard of Earthsea");
+        Assert.Contains(results, b => b.Title == "The Tombs of Atuan");
+    }
+
+    [Fact]
+    public void FindByAuthor_UnknownAuthor_ShouldReturnEmpty()
+    {
+        var results = _collection.FindByAuthor("Nobody Known");
+        Assert.Empty(results);
+    }
+
+    // --- Persistence round-trip ---
+
+    [Fact]
+    public void AddBook_ShouldPersistAcrossReload()
+    {
+        _collection.AddBook("Neuromancer", "William Gibson", 1984);
+
+        var reloaded = new BookCollection(_tempFile);
+        var book = reloaded.FindBookByTitle("Neuromancer");
+
+        Assert.NotNull(book);
+        Assert.Equal("William Gibson", book.Author);
+        Assert.Equal(1984, book.Year);
+    }
+
+    // --- Edge cases / validation ---
+
+    [Fact]
+    public void AddBook_EmptyTitle_ShouldStillAdd()
+    {
+        _collection.AddBook("", "Some Author", 2000);
+
+        Assert.Equal(1, _collection.Books.Count);
+        Assert.Equal("", _collection.Books[0].Title);
+        Assert.Equal("Some Author", _collection.Books[0].Author);
+    }
+
+    [Fact]
+    public void AddBook_EmptyAuthor_ShouldStillAdd()
+    {
+        _collection.AddBook("Some Title", "", 2000);
+
+        Assert.Equal(1, _collection.Books.Count);
+        Assert.Equal("Some Title", _collection.Books[0].Title);
+        Assert.Equal("", _collection.Books[0].Author);
+    }
+
+    [Fact]
+    public void AddBook_DuplicateTitles_ShouldAllowBoth()
+    {
+        _collection.AddBook("Same Title", "Author One", 2000);
+        _collection.AddBook("Same Title", "Author Two", 2001);
+
+        var matches = _collection.Books.Where(b => b.Title == "Same Title").ToList();
+        Assert.Equal(2, matches.Count);
+    }
 }