To start learning Microsoft ASP.NET 2.0 AJAX Extensions 1.0
it is strongly recommend that you start reading the AJAX documentation. This
should be your first reference. In addition, you can visit the official website of ASP.NET and download
all of the videos there created by Joe Stagner on the AJAX and AJAX Toolkit.
Finally, there are several MSDN web casts that are worth watching. Rob Bagby,
Microsoft Developer Evangelist, lately delivered a series of web casts on the
Client Script side of AJAX. To find out more, please check out his blog.
In addition, a new book called "Beginning ASP.NET 2.0 AJAX" by WROX is extremely recommended and can be reached here.