It's a very brain-friendly introduction to Ajax. You'll gulp all the basic stuff in no time, as it's very easy and quick to read. You'll learn about asynchronous requests, how to send and receive data, how to play with the DOM...
Some people complain that it contains lots of repetition. The learning content is not huge, but it will stick in your brain If you're a fan of the Head First series, you'll fell at home. Those complaining were probably looking for some more technical books.
If you're looking for some hard stuff, you'd better look at Ajax In Action. If all you need is a good intro to Ajax, and to get ready to code in no time, then you can read Head Rush Ajax.
You can check out a sample chapter at O'Reilly's homepage.