I usually code backends, but have been lucky enough to be allowed to try out Grails for a small webbased client. Since this is supposed to be a quickly made client and my experience with Grails is primarily listening in on the odd conference talk (you know, the talk held in the period you did not find a project relevant talk , I need some good references. Many of the links Google turn up are beginners/introduction pointers. Most of the links seem to lack either my need for howto's or plain lookup's. Does the latest promoted book (or any other books for that matter) provide both? Or, does there exist good sites on the Net I haven't found yet?
If you are looking for more than an intro to Grails book, I recommend The Definitive Guide to Grails 2nd Edition. The grails.org site is pretty informative and there are many blog posts that help explain specific topics like security and 3rd party integration.
For starters I used the Grails tutorials and documentation online but from my personal experience once you get into the details and customization you are pretty much on your own. I did not see a lot of complex test cases or examples listed in any book or online
I like the IBM Developer works series on Grails, by Scott Davis. Those are a pretty good intro type reference (from the few I've read/gone through).
But everything I've heard about this book is great. And Grails in Action is good, too.
I have to agree with Andrew on the IBM series by Scott Davis. I used them to train the Java developers on a team I was leading before writing GQuick. And if you've heard Scott speak before, when you read his articles it'll seem like he's in the room with you.