What I especially liked about this book is that it goes deep into the details if needed. For example, differences between web browsers are described in detail. The book isn't just a cookbook that explains step by step how to build an Ajax web application - it focuses on making robust and secure applications that will work well on the different browsers and operating systems that are out there.
I would highly recommend this book to people who are developing serious Ajax web applications. This book contains a lot of valuable information, I certainly learned a lot by reading it.