This is another one of those "Sniper Topics" that I like, again from PAKT Publishing.
Microsoft AJAX Library Essentials: Client-side ASP.NET AJAX 1.0 Explained: Cristian Darie,Bogdan Brinzarea: Books
Not only does this text enumerates the namespaces in the Microsoft AJAX Client Libraries, and provide good coverage of the Client Component Framework, but it teaches all the "whys" along with the "How Tos".
If your building AJAX applications and Microsoft's AJAX technology is at the center of your work then this book is a "must read".
Click on the book image above to check it out on Amazon.com