Let me first preface this review by saying this is the first technical book that I've read cover to cover TWICE prior to posting a review. I had to make sure the stuff stuck, because the material covered in Manning's very excellent "Ajax in Action" is really deep. But bringing the next evolution of user experience, giving your web applications a rich client feel, isn't completely easy. This won't scare you away from using Ajax in your existing applications, but make you aware of exactly what to expect.
The book also isn't a "copyist's" title, one that can provide working code right out of the gate. Also, the audience for this work should be fairly sopisticated and experienced with modern-day web programming, as the book assumes a certain level of competency and doesn't waste time with rudimentary concepts or examples. Crane and Pascarello take a platform-agnostic look at incorporating Ajax-style programming into web applications, citing examples in PHP, Java and .NET, and accordingly the examples are all partial and abstracted, to be implemented in whatever platform the developer/reader is familiar with.
Ajax programming is a lot more complex than it lets on, but not as daunting as you might think. This book is critical in your understanding of how to make the next big thing in web development to work for you. A must-have.