Looking for an ASP.NET AJAX book? This one has been massively updated from its 2.0 version, to cover in detail the features added in 3.5, and not to be forgotten, 3.5 SP1. For example, Script Combining was a new feature in 3.5 SP1, and so was History support, so it's an important distinction! The 2.0 book was 307 pages, and this one is 552. History support, by the way, has a dedicated chapter. Script combining is a major portion of the chapter on the ScriptManager.
Another important difference between the 2.0 and 3.5 versions -- this time, I'm a contributing author. :) My first major technical publication, hopefully more to come. I'm that weird guy on the right.