http://bulletproofajax.com
It's short, succinct, and explains what Ajax is and how to use it in very clear, easy to understand language. I was confused by the whole concept until I read this book and I was then able to build a Web application that runs essentially on one page. It's got multiple forms for updating various related parts of a database and absolutely no reloading of the page. Just very small updates to targeted sections of the page. I didn't need the 600 page tome on every little aspect of Ajax. This little book got me moving quickly into implementing Ajax. I can't say enough good about it. He doesn't go into jQuery, only raw javascript. And I think because of that, the jQuery function syntax was a lot easier to understand.
In addition to teaching the fundamentals of Ajax, what's really helpful is he explains it in the context of progressive enhancement web design so that your web pages are not dependent upon javascript being enabled.
He's got several other books I really like too. None of them are more than 150 pages and don't cost an arm and a leg (or require huge muscles to tote around). Very refreshing.
If you buy it online, look for the used editions. They have all the same words at a fraction of the cost. I got it for less than $10 in total.
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