I've been playing around with Ajax a bit. The book "Ajax in Action" is very good by the way. A very in depth treatment of the subject. The authors are Dave Crane, Eric Pascarello and Darren James. This a very detailed book. The authors not only go into detail about the technology behind Ajax but give a pretty good treatment on some common design patterns and how Ajax fits.
Here comes the but part. Although the book is very good, I not sure Ajax is quite ready for prime time. There is a lot involved with coding a simple client. A more complex client, which is the promise of Ajax, would be difficult at best. I think it will eventually come along but tooling is going to be needed for widespread adoption. I think that will be coming in 2006 but until then get ready to get in the trenches if you want to do Ajax.