Hi! Though I may be a bit biased I'd suggest my own ESSENTIAL GWT, but not as your first book. Personally, I started with the GWT documentation, but then went on by myself. My book is geared toward programmers who already have GWT knowledge, and focuses on more advanced themes; for example, I show how to use GWT 2's new UIBinder, but not how to use widgets.
Hope this helps!
Great fan of Open Source, Linux, and web development with GWT; all of these come together for my ESSENTIAL GWT book!