Matt Gibbs and I recently released a new book for WROX titled Professional ASP.NET 2.0 AJAX that covers all the topics you need to know to build robust ASP.NET AJAX applications.  Matt is a development manager for ASP.NET (and led the ASP.NET AJAX team) and I've had the opportunity to be involved with the product since the early alphas. We put a lot of work into the book so hopefully everyone finds it useful and educational....don't let the cover scare you away. :-) 

Here's the book's outline:

• Chapter 1: Developing Next-Generation Web Applications
• Chapter 2: Partial Page Updates
• Chapter 3: JavaScript for the ASP.NET Developer
• Chapter 4: Understanding the ASP.NET AJAX Client Library
• Chapter 5: Using the ScriptManager
• Chapter 6: ASP.NET AJAX Networking
• Chapter 7: ASP.NET AJAX Application Services
• Chapter 8: Rich AJAX Toolkit Controls
• Chapter 9: Testing and Debugging ASP.NET AJAX Applications
• Chapter 10: Deploying ASP.NET AJAX Applications
• Chapter 11: Building Custom Controls