Three years ago, when I first began developing with Grails, I benefited from a book by Jason Rudolph called "Getting Started with Grails" The book was well written, short, and available as a free PDF. I recommended it to everyone interested in Grails. But, as Grails rapidly improved, the book began to become outdated.
Jason Rudolph teamed up with Scott Davis (author of "Groovy Recipes") to update the book. It now covers Grails 1.2 and, once again, I highly recommend it.
You can download the book from:
But note that InfoQ will ask you to register with their site before you can download the PDF. But it is well worth it as InfoQ is a great site with lots of technical information, in form of articles and videos, on application development.