Rating: 

This book does a nice job of introducing the basics of Ajax and then progressing into how Rails has baked helpers right into the framework.

There is some duplication of coverage found in other books, but this is not necessarily a negative since not everyone owns every book.</>

The book includes three non-trivial examples with the code listed in the back. Unfortunately, though there are references to the code being available on the O'Reilly site, the code has not been loaded as yet.

If you are looking for the most in-depth book on Ajax as it relates to Rails, this is your only option. Other books have chapters or sections, but this book is exclusively about this top. You could find information about this topic online, but not organized so well, and not written so well. As such, definitely worth the price (IMHO).