Introducing Microsoft ASP.NET AJAX

So far I think every book or article I have read by Dino Esposito has been great, this is no exception!
This book is broken up into three parts:

• The building blocks of Ajax
• Adding Ajax to an existing site
• Client-Centric Development

Oh yeah, and some bonus content at the end!
Overall this book is an excellent starting place for getting into Ajax development or to have a reference.