Ajax Patterns and Best Practices enables you to just pick up the book and then write applications that work properly. This book is not just about the technical, low-level details of the API's, but is about making something happen on both the client and server. For example, the Ajax Widgets chapter outlines a number of canned widgets that perform some functionality. The book explains the widget implementation so you can not only understand what is happening, but copy and paste if necessary.
The book also covers the server side with the REST protocol. REST and Ajax dovetail elegantly with each other, but REST can also be used solo, with just a computer-to-computer solution. And like Ajax, REST can be used with today's existing technologies. Currently, millions of client computers are Ajax-ready, and mullions of servers are REST-ready.
The book is suitable if you're a beginner, or if you have already created an Ajax application. Throughout the book, various patterns and best practices are outlined so that if you are more experienced, you can just glance and verify that you are building the most efficient Ajax application possible.