I've recently received a complimentary of the Professional ASP.NET 3.5 Ajax book by WROX authored by several big names in .NET Web Development, such as Bill Evjen, Matt Gibbs, Dan Wahlin and Dave Reed. Of this list, Matt's from the team that developed the technology itself.
I was very surprised to find this book so well written for all levels of readers, with a very good description of AJAX history and concepts as well as more indepth topics. Certainly a great book to add onto your book shelve if you want to get up to speed with ASP.NET Ajax fast.