The Ajax world moves really quickly, and has moved on a lot since the publication of this book, so much so that it could really do with a new edition. Fortunately, with the sub title "A Web 2.0 Primer", we should expect an overview, and in some ways it doesn't matter that we don't get the most up to date information.
The authors start by explaining a little about Ajax architecture. And an example of creating a very simplified Google Maps which I think I would have preferred as an appendix as it didn't really deliver an Ajax app, but I did learn a little about png that I didn't know.
"Chapter 3 - Ajax in Action" and we really start to learn some Ajax as we get to see the XMLHttpRequest in action. A quick overview of cross-browser compatibility opens our eyes to the fact that we should probably find a decent Ajax library and use that.
Unfortunately for the book, more frameworks exist now, than did at the time of publication, and the ones mentioned have advanced so you can read the coverage in this book as 'libraries do things like this' and then you have to go hunt the web and consult ajaxian.com to find out the new libraries that 'everyone' uses.
The chapter on Debugging helped but, again, the debuggers mentioned have all advanced and new ones have appeared e.g. GreaseMonkey techniques, FireBug. I did not know about Venkman, Mochikit, or View Source Chart, so I did learn something.
Then an examination of some server-side integration frameworks: JSON-RPC, SAJAX (PHP), Ruby on Rails, DWR (Java), Ajax.Net, and Atlas (.Net).
The book ends with a quick overview of the (then) new Canvas, and SVG.
A pretty quick book to read. Which you immediately want to supplement with some good web searches, but with sadly few follow on links in the book. The book's web page discussion area has very little information but downloading the book's code might prove interesting.
Just in case the above seems overly negative. At the same time as I read Pragmatic Ajax, I also read "Ajax: Your visual blueprint for creating rich internet applications", and I much preferred Pragmatic Ajax.