If you haven't watched Joe's videos before, I highly recommend checking them out. They are short, focused, videos that drill in on a specific topic and are designed to help master common scenarios when using the product. Both VB and C# sample code versions can be downloaded for each video topic.
You can watch all of the videos online for free at: http://www.asp.net/learn/videos/ (also make sure to check out the more than 50+ other videos on that page as well).
Now that Joe has finished covering a lot of the ASP.NET AJAX and ASP.NET AJAX Control Toolkit basics, he is starting on a new "ASP.NET AJAX Patterns" series that will go deeper and discuss broader patterns to think about with AJAX (for example: predictive fetching, content chunking, page rearrangement, etc). Make sure to subscribe to the ASP.NET Videos RSS Feed to keep an eye on this series as future videos are released.
ASP.NET AJAX in Action Book
Alessandro "Garbin" Gallo, David Barkol, and Rama Krishna Vavilala have also been working on what is shaping up to be a great ASP.NET AJAX 1.0 book for Manning called "ASP.NET AJAX in Action":
Hope this helps,