The second edition of Grails In Action is already in "early access" and is due to be published later this year. I found the first edition to be a great introduction to working with Grails, although it's a bit out of date now.
You don't need to learn Groovy first, as it's pretty easy to pick up some Groovy while you're learning Grails, if you already know some Java. But it's definitely worth learning Groovy properly anyway, as it's useful outside Grails e.g. in tools like Gradle, or for scripting with Java libraries. I recommend "Groovy In Action" by Dierk Konig as a good resource.
There are also lots of tutorials on the web, which may be more recent than these books, as well as a series of articles on Groovy and Grails at the IBM developerWorks website.
I second the recommendation of Grails in Action 2nd Ed. Also, I would pickup the 2nd Ed. of Groovy in Action as a companion guide for some finer groovy points a pure Java Developer would miss. This edition is in MEAP and is due to be published in the next month.
Another nice thing is http://groovy.codehaus.org/ has very good documentation with plenty of coding examples to help with the groovy and follow Guillaume Laforge blog at http://glaforge.appspot.com/ can provided valuable nuggets to learning grails.
"Your work is going to fill a large part of your life, and the only way to be truly satisfied is to do what you believe is great work. And the only way to do great work is to love what you do." -- Steve Jobs
My book, available in print just this week, is "Making Java Groovy", http://manning.com/kousen . It's much more of a Groovy book than a Grails one, but the chapter on web development does have a nice Grails example. Ultimately you will need some background in Groovy, though. While I feel compelled to recommend my book, Groovy in Action (http://manning.com/koenig2) is still one of my favorite technical books.
I'll be here answering questions all week, officially starting tomorrow. I just thought I'd get a jump on it now.