I recently got a copy of Microsoft AJAX Library Essentials: Client-side ASP.NET AJAX 1.0 Explained  by Cristian Darie  and  Bogdan Brinzarea image.    I think it is very, cool that there is a whole book on the Microsoft Ajax Library (download)... I think this is a far two unsung asset of the ASP.NET 3.5

The first half of the book is really an Ajax and javascript overview.. the big take away from this section is that you DO NOT want to be doing this stuff by hand.  You gotta use a framework.  Of course, I am slightly partial to the Microsoft Ajax Library, put Dojo, Prototype, or any of the many others would be better than doing this by hand!

The second half of the book is a good walk through of the Microsoft Ajax Library.. how to do basic networking, create classes and components, etc.  The authors also do a great job of highlighting the tools of the trade for Ajax developers including Nikhil's web developer helper and of course FireBug.  The book also includes a handy class reference in the appendix.

I think the book misses out on talking about how to use the Ajax Control Toolkit completely from client side javascript, but maybe that is for the 2nd edition ;-)