Quick summary: Chapters 5-7 worth the cover price. Rest of the book could be shortened, and replaced with more of chapters 5-7.
Chapter 1 is a history of the Universe through the lens of distributed programming. As a person who has tried to deliver that story in front of a live audience, I can readily attest that such a history is often lacking in detail and perceived as redundant by the audience. This chapter suffers from those problems, and while interesting, would have been better as a prolog than a whole chapter.
However, there are some problems. The code presented in these chapters is brittle. The authors gloss over some rather important complexities. I'd give them a pass if they were then introducing frameworks that took care of the complexities for you; but since they don't, it would have been nice for the code to go into more detail. Examples are ignoring the MSXML version of XHR when showing cross-platform invocation, failing to account for multiple concurrent asynchronous requests, and robust error handling. In the last case, the authors show how to inspect the status code of the XHR response, but don't examine issues like request timeouts and corrupt data.
Chapter 4 really starts to pick up. This chapter introduces a ton of Ajax techniques in code format. It shows, in detail, how to implement your own Ajax effects like auto-complete fields, page polling, interacting with web services, and many more. The code is useful, but perhaps more useful is the effect of seeing so many techniques laid out for you, expanding your idea of what is possible in your web application. Very well done.