The book begins with a brief review of AJAX architecture with an emphasis on security. The writing style is quite engaging including a chapter walking you through an attack from a hacker's point of view. All the major known categories of attacks are included including resource enumeration, parameter manipulation (with SQL and XPATH injection), session hijacking, JSON hijacking, XSS, CSRF, phishing, denial of service, etc.
I particularly liked the analogies to things that happen in the physical world such as resource injection into a roommate's "to do" list and hijacking another customer's paid order in the deli. These made it easy to visualize the problem even for people who don't code often.
All information was up to date as of printing including comments on all four major browsers (IE, Firefox, Opera and Safari.) They even mentioned the HTML 5 specification. The book is not server side language specific, which was nice.
It's written in a style that makes it easily approachable, and complex topics are explained well. Although some of the later material assumes knowledge of the earlier stuff, most chapters can be skipped if the reader isn't interested in a particular topic, and revisited later. I recommend the book to every web professional.