The format of the book is long code examples with follow-up discussion and tips scattered throughout. This makes for interesting reading and helped me discover quite a few new techniques. The only downside is that the discussion of some features is spread out over different examples and chapters, which made it easy to forget where I found something and difficult to focus in on help with a specific feature. The examples are written using version 1.3.3 of GWT, but the authors note areas where the upcoming 1.4 version fixes bugs and improves on functionality.
David Geary has also written one other GWT book (e-book only), called Google Web Toolkit Solutions Cool & Useful stuff.