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.