Okay, a short blog post like this (or even a big one, like those penned by Steve Yegge) can't tell you everything *known today* about Ajax, forget "all you ever need to know". In fact, it can't tell you everything about anything worth knowing. There is just way too much information and knowledge around us about almost everything, consequential or not. To make things worse, at least for those who claim to "tell everything", this body of information and knowledge keeps growing every minue.
So why did I choose this particular title? No, I didn't intend to write everything I know about Ajax. It is just a link-bait. Seems to have worked quite well for others. Might work for me as well.
On the other hand, the book does provide good introduction to basic concepts, is quite readable, includes a lot of source code for non-trivial working programs and lists relevant resources, such as Ajax libraries, frameworks and applications, in its References section. I especially liked the "chat" and "whiteboard" application that allows two or more users to share a whiteboard and chat through their browsers.
Okay, so how does this book compares with other books on the same topic? This is a tough question, for I haven't been paying attention to most books that have come out on this topic. Though there is a answer, and it comes from this Amazon Sales Rank comparison chart:
A higher Sales Rank for an item implies that more people are buying it from Amazon. This doesn't tell how well a particular book will meet your needs but just that the high ranking items, in general, are being bought by more people than the low ranking ones. The above chart does indicate that Ajax -- The Definitive Guide is outselling its rivals, at least at the time of this review (March 17-18, 2008).