I actually bought a book at Borders today, something I never do anymore. Computer books in particular are rare purchases for me because there are few that I need or want to learn from.
If you've used Gmail, you've used a Web app that uses Ajax. Ajax stands for asynchronous JavaScript and XML, which really just has to do with a method of having your Web page exchange data with a server without refreshing the entire page. These technologies have been around for a long time, it's just that they're finally be used together in really interesting ways. There's quite a bandwagon behind this stuff, much of it hype. Yes, browser-based apps are all the rage, and have been around for a long time, but there are now design patterns you can clearly identify and other people have made mistakes for you.
Ajax in Action looks at the state of the combined technologies and how they relate, then goes right into the patterns that make it work. It even covers security issues and various frameworks out there. The examples are pretty solid too, especially the type-ahead suggest (auto complete) example. Neat stuff. The single most useful thing I've read so far is an appendix about JavaScript for OO code monkeys. I read that first, and it sure clears up a lot of things about JavaScript I didn't realize.
Do I have use for any of this? Well, I can think of a couple of instances where it may be useful to me. I guess the reason that I'm drawn to it is just that it's something new.