src/NexusReader.Application/Features/Books/Commands/PublishBookVersionCommandHandler.cs

@@ -38,7 +38,7 @@ public class PublishBookVersionCommandHandler : ICommandHandler<PublishBookVersi
 
         if (book == null)
         {
-            throw new BookNotFoundException(request.BookId);
+            return Result.Fail(new Error($"Book with ID '{request.BookId}' was not found."));
         }
 
         var oldDraftRevision = book.CurrentDraftRevision;

src/NexusReader.Application/Queries/Creator/GetBookRevisionsQuery.cs

@@ -41,7 +41,7 @@ public class GetBookRevisionsQueryHandler : IQueryHandler<GetBookRevisionsQuery,
 
         if (!bookExists)
         {
-            throw new BookNotFoundException(request.BookId);
+            return FluentResults.Result.Fail<List<CreatorBookRevisionDto>>(new FluentResults.Error($"Book with ID '{request.BookId}' was not found."));
         }
 
         // Fetch all revisions sorted chronologically

src/NexusReader.UI.Shared/Components/MarkdownEditor.razor

@@ -43,6 +43,7 @@
     private IJSObjectReference? _module;
     private DotNetObjectReference<MarkdownEditor>? _dotNetHelper;
     private string? _lastInitializedEditorId;
+    private bool _disposed;
 
     private enum SaveStatus
     {
@@ -350,6 +351,7 @@
 
         // Cancel pending timers thread-safely
         CancellationTokenSource? ctsToCancel = null;
+        CancellationToken token;
         lock (_timerLock)
         {
             if (_debounceCts != null)
@@ -358,6 +360,7 @@
                 _debounceCts = null;
             }
             _debounceCts = new CancellationTokenSource();
+            token = _debounceCts.Token; // Capture token synchronously under lock on UI thread
         }
 
         if (ctsToCancel != null)
@@ -376,13 +379,6 @@
         // Start 5-second idle debounce timer
         _ = Task.Run(async () =>
         {
-            CancellationToken token;
-            lock (_timerLock)
-            {
-                if (_debounceCts == null) return;
-                token = _debounceCts.Token;
-            }
-
             try
             {
                 await Task.Delay(5000, token);
@@ -401,7 +397,7 @@
 
     private async Task TriggerAutosaveAsync(string markdown, CancellationToken token)
     {
-        if (token.IsCancellationRequested) return;
+        if (token.IsCancellationRequested || _disposed) return;
 
         _status = SaveStatus.Saving;
         await InvokeAsync(StateHasChanged);
@@ -416,6 +412,8 @@
                 token
             );
 
+            if (_disposed) return;
+
             if (response.IsSuccessStatusCode)
             {
                 // Purge LocalStorage backup key on HTTP success
@@ -431,10 +429,12 @@
         }
         catch (Exception ex)
         {
+            if (_disposed) return;
             _status = SaveStatus.OfflineLocalBackup;
             Console.WriteLine($"[MarkdownEditor] Autosave HTTP exception: {ex.Message}");
         }
 
+        if (_disposed) return;
         await InvokeAsync(StateHasChanged);
     }
 
@@ -477,6 +477,7 @@
 
     public async ValueTask DisposeAsync()
     {
+        _disposed = true;
         try
         {
             _cts.Cancel();

src/NexusReader.Web/Program.cs

@@ -519,18 +519,15 @@ app.MapGet("/api/creator/books/{bookId:guid}/revisions", async (Guid bookId, Cla
 
     var tenantId = user.FindFirstValue("TenantId") ?? "global";
 
-    try
-    {
-        var result = await mediator.Send(new NexusReader.Application.Queries.Creator.GetBookRevisionsQuery(bookId, userId, tenantId));
-        if (result.IsSuccess) return Results.Ok(result.Value);
+    var result = await mediator.Send(new NexusReader.Application.Queries.Creator.GetBookRevisionsQuery(bookId, userId, tenantId));
+    if (result.IsSuccess) return Results.Ok(result.Value);
 
-        var errorMsg = result.Errors.Count > 0 ? result.Errors[0].Message : "Unknown server error";
-        return Results.BadRequest(errorMsg);
-    }
-    catch (NexusReader.Domain.Exceptions.BookNotFoundException)
+    var errorMsg = result.Errors.Count > 0 ? result.Errors[0].Message : "Unknown server error";
+    if (errorMsg.Contains("was not found", StringComparison.OrdinalIgnoreCase))
     {
-        return Results.NotFound($"Book with ID '{bookId}' was not found.");
+        return Results.NotFound(errorMsg);
     }
+    return Results.BadRequest(errorMsg);
 }).RequireAuthorization();
 
 app.MapPost("/api/creator/books/{bookId:guid}/publish", async (Guid bookId, [FromQuery] string version, ClaimsPrincipal user, IMediator mediator) =>
@@ -545,18 +542,15 @@ app.MapPost("/api/creator/books/{bookId:guid}/publish", async (Guid bookId, [Fro
         return Results.BadRequest("Version string is required.");
     }
 
-    try
-    {
-        var result = await mediator.Send(new NexusReader.Application.Features.Books.Commands.PublishBookVersionCommand(bookId, version, userId, tenantId));
-        if (result.IsSuccess) return Results.Ok();
+    var result = await mediator.Send(new NexusReader.Application.Features.Books.Commands.PublishBookVersionCommand(bookId, version, userId, tenantId));
+    if (result.IsSuccess) return Results.Ok();
 
-        var errorMsg = result.Errors.Count > 0 ? result.Errors[0].Message : "Unknown server error";
-        return Results.BadRequest(errorMsg);
-    }
-    catch (NexusReader.Domain.Exceptions.BookNotFoundException)
+    var errorMsg = result.Errors.Count > 0 ? result.Errors[0].Message : "Unknown server error";
+    if (errorMsg.Contains("was not found", StringComparison.OrdinalIgnoreCase))
     {
-        return Results.NotFound($"Book with ID '{bookId}' was not found.");
+        return Results.NotFound(errorMsg);
     }
+    return Results.BadRequest(errorMsg);
 }).RequireAuthorization();
 
 app.MapPost("/api/creator/books", async (

tests/NexusReader.Application.Tests/Commands/PublishBookVersionTests.cs

@@ -169,7 +169,7 @@ public class PublishBookVersionTests : IDisposable
     }
 
     [Fact]
-    public async Task Handle_WithMismatchedTenantId_ThrowsBookNotFoundException()
+    public async Task Handle_WithMismatchedTenantId_ReturnsFailure()
     {
         // Arrange
         var bookId = Guid.NewGuid();
@@ -210,13 +210,16 @@ public class PublishBookVersionTests : IDisposable
 
         var handler = new PublishBookVersionCommandHandler(_dbContextFactoryMock.Object);
 
-        // Act & Assert
-        var action = () => handler.Handle(command, CancellationToken.None);
-        await action.Should().ThrowAsync<BookNotFoundException>();
+        // Act
+        var result = await handler.Handle(command, CancellationToken.None);
+
+        // Assert
+        result.IsSuccess.Should().BeFalse();
+        result.Errors.Should().Contain(e => e.Message.Contains("was not found"));
     }
 
     [Fact]
-    public async Task Handle_WithMismatchedUserId_ThrowsBookNotFoundException()
+    public async Task Handle_WithMismatchedUserId_ReturnsFailure()
     {
         // Arrange
         var bookId = Guid.NewGuid();
@@ -257,13 +260,16 @@ public class PublishBookVersionTests : IDisposable
 
         var handler = new PublishBookVersionCommandHandler(_dbContextFactoryMock.Object);
 
-        // Act & Assert
-        var action = () => handler.Handle(command, CancellationToken.None);
-        await action.Should().ThrowAsync<BookNotFoundException>();
+        // Act
+        var result = await handler.Handle(command, CancellationToken.None);
+
+        // Assert
+        result.IsSuccess.Should().BeFalse();
+        result.Errors.Should().Contain(e => e.Message.Contains("was not found"));
     }
 
     [Fact]
-    public async Task Handle_WithNonExistentBook_ThrowsBookNotFoundException()
+    public async Task Handle_WithNonExistentBook_ReturnsFailure()
     {
         // Arrange
         var command = new PublishBookVersionCommand(
@@ -275,9 +281,12 @@ public class PublishBookVersionTests : IDisposable
 
         var handler = new PublishBookVersionCommandHandler(_dbContextFactoryMock.Object);
 
-        // Act & Assert
-        var action = () => handler.Handle(command, CancellationToken.None);
-        await action.Should().ThrowAsync<BookNotFoundException>();
+        // Act
+        var result = await handler.Handle(command, CancellationToken.None);
+
+        // Assert
+        result.IsSuccess.Should().BeFalse();
+        result.Errors.Should().Contain(e => e.Message.Contains("was not found"));
     }
 
     public void Dispose()

tests/NexusReader.Application.Tests/Queries/CreatorDashboardTests.cs

@@ -234,7 +234,7 @@ public class CreatorDashboardTests : IDisposable
     }
 
     [Fact]
-    public async Task GetBookRevisions_WithMismatchedUserOrTenant_ThrowsBookNotFoundException()
+    public async Task GetBookRevisions_WithMismatchedUserOrTenant_ReturnsFailure()
     {
         // Arrange
         var userId = "creator-123";
@@ -262,12 +262,14 @@ public class CreatorDashboardTests : IDisposable
 
         // Act & Assert
         var queryMismatchedTenant = new GetBookRevisionsQuery(bookId, userId, "different-tenant");
-        var actionTenant = () => handler.Handle(queryMismatchedTenant, CancellationToken.None);
-        await actionTenant.Should().ThrowAsync<BookNotFoundException>();
+        var resultTenant = await handler.Handle(queryMismatchedTenant, CancellationToken.None);
+        resultTenant.IsSuccess.Should().BeFalse();
+        resultTenant.Errors.Should().Contain(e => e.Message.Contains("was not found"));
 
         var queryMismatchedUser = new GetBookRevisionsQuery(bookId, "different-user", tenantId);
-        var actionUser = () => handler.Handle(queryMismatchedUser, CancellationToken.None);
-        await actionUser.Should().ThrowAsync<BookNotFoundException>();
+        var resultUser = await handler.Handle(queryMismatchedUser, CancellationToken.None);
+        resultUser.IsSuccess.Should().BeFalse();
+        resultUser.Errors.Should().Contain(e => e.Message.Contains("was not found"));
     }
 
     public void Dispose()