I flipped through ASP.NET AJAX in Action and Professional ASP.NET 2.0 AJAX this weekend. Both looked look great books for anyone that wants to go beyond the basics.
I found this http://www.amazon.com/dp/1847190987?&camp=212361&creative=380789&linkCode=wsw&tag=computerprogrammingbooks-20
book which specifically says"This book is written for ASP.NET developers who not only want to understand how Microsoft AJAX will impact on the way they develop web applications, but also want to make the transition to better, more responsive applications as
easily as possible. You will need access to either Visual Studio Web Developer Express or Visual Studio .NET 2005.
All the code examples are in C#."