Pro Web 2.0 Application Development by Jeff Dwyer is the first book on professional Web 2.0 principles that includes the Google Web Toolkit (GWT). GWT is one of the leading Java Ajax frameworks in market, and is very hot in terms of relative growth; it can be integrated with other broader Java frameworks like JBoss Seam and possibly Spring. The book's focus on a single, extremely rich, example "killer application" is what sets this apart from other GWT titles.
This book is most suited to those who have already read an introductory book on GWT and have cobbled up their first GWT learning project. They've got the basics but want to see how best to use GWT 1.6 in a professional, enterprise scale web application.
This book explains the internal workings of a sample, illustrative project called ToCollege.net whose source code is freely available online. The idea here is to learn by studying a real-world application. There is some introductory material but most of this book is about integrating GWT with Spring, Acegi, FreeMarker, SiteMesh, Hibernate, Google Maps, and Google Gears (yea, the book is a little dated now).