I've read most of the WPF books on the market at the moment and like
Tim Sneath mentions they all take a different approach.
The
Chris Sells and Ian Griffiths book came early in the beta cycle and was a very welcome at the time.
Next
Charles Petzold's Microsoft Press book, Applications = Code + Markup took a very different route, with half the book on code and half on markup. For me a book on graphics with no pictures made the content seem quite dry.
I'm currently reading
Adam Nathan's Windows Presentation Foundation Unleashed - it's in full colour, loads of tips and pointers throughout the chapters - it's a great read and definately my recommendation.

Also it seems like
Chris Anderson has finished his book now, which I'll take a look at when it is out.
Adam Bosworth, ex-Microsoft, now Google employee talks about
why AJAX failed but is now all the rage.
I do agree with his point of simple web applications - for me Amazon is still one of the 'easiest' sites to use without lots of AJAX/Flex/etc., in it's case the ease of use and content are king.
For instance, Bosworth said a cardinal rule of his is KISS, or, in
his words, "Keep It Simple and Stupid." Gestures like tooling, icons,
right-click and drag-drop are too obscure, he said.
Moreover, most Web applications are designed for large numbers of
customers with small amounts of customer support, and most Web applications are
not used for hours a day, he said.
I've been using the
Microsoft AJAX libraries (formerly codename Atlas) (including the Futures CTP and Control Toolkit).
After the pain of keeping up with all the CTP, RC1 changes to tags and web.config - it is delivering on improved user experience for our projects with little effort by leveraging the
UpdatePanel.
Documentation is a welcome compared to other AJAX frameworks and
JSON webservice calls are well supported to.
If you are happy to mix query syntax along side traditional coding, then
LINQ against DataSets is looking powerful.