If you are looking to use appropriate technology for your web development rather than just looking at adding the latest gee-whiz technology whether it is appropriate or not then this may be the best book on Ajax for you.
Unlike most of the other recently written books about Ajax, this one doesn't just look at how to use Ajax, it also looks at what Ajax can be used to do and when it is an appropriate technology to use.
The book starts out by looking at what Ajax actually is and what it is useful for. It looks at how Google used it to implement Google Maps (one of the early useful Ajax implementations) and shows you how to create your own equivalent code. It then moves on to look at some of the more popular frameworks that can be used to add Ajax functionality and more to your web pages without your having to write your own code from scratch. It also looks at some of the popular server side technologies one at a time allowing you to see where this important aspect of Ajax fits into the picture (something skipped over or mishandled in many other Ajax books).
The emphasis of this book is (as its name suggests) on how to use Ajax appropriately and practically as one of the tools in your web development toolbox. The code in this book are practical uses for the technology and by the time you reach the end of the book you should have not only a clear idea of how to use Ajax but also when to use it (and perhaps more importantly - when not to use it).