The Pragmatic Programmers (the guys who wrote the fairly famous book of almost the same name) are releasing a book on AJAX techniques. While it’s only two-thirds finished, they’ve released it as a beta book, which essentially means that you can pay for the bundle of the PDF and printed version of the book now, and get the PDF of it now, with updates throughout the process.
I’ve found these books in general to be of the highest quality (I’ve read a few, including Agile Programming with Rails; The Pragmatic Programmer; Pragmatic Unit Testing and Programming Ruby, among others, and never been disappointed. I did the beta book program with Agile Programming with Rails, and was very pleased with the whole process.
I’ve skimmed this book through and read the first two chapters, and it looks to continue their tradition of high-quality text about a cutting-edge subject with little formal documentation elsewhere. Highly recommended.