This book is a nice introduction to GWT which is suitable for beginners. I myself being a beginner in GWT found this book as a good starting point to learn GWT. One thing you should know however is that this book covers Ext GWT which can be confusing at first.
The book starts from the basics of creating a GWT project using Netbeans and moves to creating layouts, forms, handling different events, making Ajax calls and deploying a GWT application. Sometimes it feels like there is too much code and very little explanation of how the code is actually working.
The book has many flaws like repetition of similar recipes, and the focus of the book is not entirely on GWT. There are chapters on database design, JPA, JasperReports which is sort of unnecessary. Instead the book could have covered things like how to create a Grid and populating it using GWT. That sort of information would've been more useful.
All said this book is one of the very few beginner level books which covers GWT 2. In spite of its flaws the book can be useful for beginners in GWT.
Disclosure: I received a copy of this book from the publisher in exchange for writing this review on behalf of CodeRanch.