Ajax in Action is a good book. And even though its server-side examples are not Java-focused, that's not all that important for learning Ajax, which is all client-side technology and is independent of what's being used on the server.
If you want more
Java focused examples, the follow-on book,
Ajax in Practice, exclusively uses Java for its back-end examples (disclaimer: I am an author of this book). While you
can start with
Ajax in Practice, as it has introductory chapters that teach the basics of Ajax, I'd recommend working through both books if you want a thorough treatment of the subject.