I started reading this book around 4pm one afternoon, and I didnt stop until 3am the next morning. I never knew Grails was so cool! I was especially excited because I had just finished a project using Struts 1.3. What a nightmare that was, and what a dream come true Grails is. I really enjoyed this book. I think it's a terrific introduction. It's short, so I think I'll need to get another book before I'm ready to start my own Grails project. I also agree that the book needs a little update since it was written for Grails version 0.3.1, and the latest version of Grails is 1.4.61.
If you're new to Grails, and you are curious about what it is, and what it can do, this is a great book. He leads you through a sample application and he shows you how to define the database tables and the joins between them, how to modify the generated jsps, how to use the Grails tag lib, how to create your own tag, how to create Hibernate Criteria objects in your project for searching, how to insert your own CSS file, how to integrate with Java pojos... all sorts of stuff. This is a great place to start with Grails.