Since the Google I/O 2009 is just about to finish I think its a great time to learn some GWT to play with different types of Google stuff (like wave!!!) and try to really understand everything there is to know on its architecture.
I'm a very fast learner from tutorials and books, but since there seem to be different areas around GWT (from how to use it, debug it, host it, etc) I think the best approach would be to read a book about it.
So what is the best book to learn everything there is to learn about GWT?