Wally McClure, Steve Smith and several others recently had their Beginning ASP.NET 2.0 AJAX book released.  I hear that lines at book stores around the world were huge and people have been camping out for days to be the first ones to get it. :-)  Congrats to everyone involved with writing it.  I haven't seen it yet, but knowing several of the authors I'm sure it's excellent.

If you're working with ASP.NET AJAX you may also want to check out the Professional ASP.NET 2.0 AJAX book that Matt Gibbs and I co-authored as well.