A couple of months ago I picked up a cope of Advanced ASP.NET AJAX Server Controls and read it cover-to-cover.
When ASP.NET AJAX was first released back in 2007 I bought Professional ASP.NET AJAX and read it cover-to-cover, so this wasn't my first foray into .NET AJAX books. But I must say that i completely pales in comparison to Advanced ASP.NET AJAX Server Controls.
Understanding the MS AJAX framework
I've done quite a bit with MS AJAX since it was first released, and thought myself reasonably adept in understanding what you can do with it, but after reading the early chapters I found out that I hadn't really scratched the surface.
Some of the things I really liked was how the authors explain the difference between Sys.Component, Sys.UI.Control, and Sys.UI.Behavior, something which I didn't fully understand prior to reading through the book.
I also really liked how they explained the MS AJAX client life cycle much better than I'd previously encountered.
How events are registered, how the internals of Sys.Application works, etc, are reason enough to buy the book.
MS AJAX and ASP.NET
When I started with ASP.NET AJAX I was like a lot of people and thought that UpdatePanel's were awesome. I've since changed my thinking but I there are places where they can still be of use. The book covers how to best work with them and pure AJAX implementations, or how your own ASP.NET AJAX server controls can avoid problems with them.
The book also looks at how to write server controls specifically designed for ASP.NET AJAX integration, and the difference between a server control and a server control behavior.
Again a concept that a lot of developers don't seem to pick up on.
If you're an ASP.NET developer who is serious about doing AJAX work then I cannot recommend this book more highly. I loaned it to one of our user interface developers at work (who was not sold on the idea of MS AJAX as a client framework) and he is now using the framework (and design pattern) in a WYSIWYG editor which he is building.
The long and the short is, buy this book.