I've recently started to learn Grails using the Mastering Grails series on developerWorks. Its not bad for a first look, but I am ready for something a little more thorough. I am thinking about buying the book Grails in Action (if I don't win one on JavaRanch!). Another problem with the Mastering Grails series is it is not up to date. It was written for Grails 1.0 and I have Grails 1.1.1. I am surprised at the extent of the differences. What version of Grails does the book use? I looked on Amazon to find out, but could find the info.
I suspect that the differences in 1.0 and 1.1.1 aren't as significant as they feel to me. I am blindly following the tutorial, without knowing why the syntax is the way it is. I think if I knew some groovy it could help...
Grails 1.1 does introduce some quite significant changes (particularly around the testing framework and the plugin infrastructure). Most of the core knowledge on how the MVC frameworks works is totally transferrable from 1.0, though.
Grails in Action is targeted to 1.1. We spent quite a deal of time updating the manuscript when the new version came out.
If you are unable to get hold of "Grails In Action" (say, if the bookstores and Amazon are all sold out after the massive demand from JavaRanch users....), Graeme Rocher's "Definitive Guide to Grails (2nd edition)" (2009) is definitely worth getting and is based on Grails 1.1, with lots of in-depth material on how Grails works.
And I can also recommend "Programming in Groovy" by Venkat Subramaniam to help you understand more about how Groovy works.