This is the second book about ajax that I have read. And what a book it is. If you want to go into the depth of ajax this is the book you want. Within this review I will explain why.

This book is devided into four parts:
Rethinking the web application
Some general discussion about website programming and your first ajax helloworld application. The interesting stuff already starts here. This is your first introduction into using design patterns in your javascript code. Excellent description of some of the more important design patterns are given. Even if you do not like ajax you can leanr something about patterns here.
Core techniques
Discusses the actual implementation of Model View Controller pattern within ajax. Some nice tips on javascript and style sheet usage.
Professional ajax
Discusse the more advanced Professional ajax side. Now we are learning about security, usability and performance. Also very interesting stuff
Ajax by example
Interesting if you want to learn how it really works, I'dd rather use an existing library, but it is fun to learn.

So this is a great book, with real live examples and a very wide coverage of the subject.