This week's book giveaway is in the OO, Patterns, UML and Refactoring forum. We're giving away four copies of Refactoring for Software Design Smells: Managing Technical Debt and have Girish Suryanarayana, Ganesh Samarthyam & Tushar Sharma on-line! See this thread for details.
I'd argue that the website is a good way to learn. Perhaps it's been updated since you made the post. Check out the Beginners Tutorial. The User Guide also has a lot of useful information. It'll save you a few bucks if you haven't already bought a book. [ October 16, 2007: Message edited by: Josh Brown ]
A good book to read if your trying to get the hang of Groovy is "Groovy Programming: An introduction for Java Developers" by Ken Barclay and John Savage - they taught me how to use Groovy and I've found it a good book.