The PDF document that I wrote with Matt Gibbs a while ago on UpdatePanel is now up to date and in sync with the RTW (Release to Web) version of the ASP.NET 2.0 Ajax Extensions. It's a 60 page document that teaches how to use the UpdatePanel control to incrementally "ajaxify" an ASP.NET 2.0 web site. The booklet comes with downloadable code samples that are immediately applicable in the real world.
The update contains a few new sections that cover the features that appeared since the last revision such as the client-side events for partial updates.
Of course, if you bought it before this final version, O'Reilly offers the current version for free.
Download the code samples: