SAMS Teach Yourself: Ajax in 10 minutes

Synopsis:

A series of short 10 minute lessons clearly explaining the core components of Ajax and how to combine them to take advantage of this powerful integrated technology.

Review:

As "Teach Yourself in 10 Minutes" books go, this is possibly the first one I've found that is close to the time!
It's aimed at intermediates+ as you should have some knowledge of php, xml, javascript.
It's got specific sections on various protocols and libraries and is ideal as a quick reference book.