This book is great and can be divided into four parts.
Part two of the book goes into detail about what is happening behind the scenes in the implementation of the UpdatePanel. When using the UpdatePanel, there has to be an instance of a ScriptManger on the ASP.NET page. At runtime, this causes an instance of a PageRequestManager object to be constructed on the client. It is this instance that manages the page requests that are passed using asynchronous calls back into the server. The viewstate for the page is threaded through these calls to keep everything consistent, limiting the system to a single active call per page.
The book then gives a brief introduction to the Ajax Control Toolkit which offers a wide range of useful components, from things like simple text box auto-completers to calendar controls to animation controls like FadeAnimation, ScaleAnimation and ConditionAnimation.
Part four of the book covers other aspects of Ajax. Support for history and declarative data binding were interesting parts of this section.
This book was good because it showed how the technology could be used by way of some interesting examples, but also went in depth as to how the technology worked, and why it worked that way.