Take this book and the ASP.NET AJAX Framework with two grains of salt, if you're serious about AJAX then learn JavaScript and look into the alternative AJAX Libraries like: jQuery, Scriptaculous, Dojo, Mootools, and YUI.
This book lacks objectivity and often suffers from hype. The authors came across as lacking proficient experience with the JavaScript language, or exposure to other AJAX Frameworks / Libraries, or sufficient experience using the ASP.NET AJAX Framework in real world situations.
Comments like: "we recommend that...", "because it makes no sense...", "you must understand X,Y,Z to run complex client-side code without writing a single line of JavaScript" were discouraging, and always left me hanging. Good books answer more than the "hows" - comments like "you must rely on a special..." are a cry for more research.
The book skims over the bigger picture of why the ASP.NET AJAX framework is implemented, or where its going. Instead it presented many examples of "how-to" implement the various ASP.NET AJAX constructs and patterns. It would have been nice to see some real world examples of why the ASP.NET AJAX client-side typing system is useful, or why the AJAX Framework's extended JavaScript objects are useful and how these contrast to what JavaScript already provides, or what happens behind the scenes when a class is registered through client-side code, or why the AJAX Framework prefers declarative syntax (XML) over imperative syntax, and to have some real in-depth discussions on performance implications.
In addition to these disappointments, the ASP.NET AJAX Framework itself has serious technical flaws, it's too heavy weighted (bloated) for practical use - aside from demos you won't see this framework used in the real world. It's a short lived framework, many of the other AJAX Libraries already offer superior performance and better user experience I was disappointed with the server-centric approach the ASP.NET AJAX Framework and this book take, and was disappointed to have JavaScript continually swept under the carpet as magic.