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 I found the first edition to be a great introduction to working with Grails, although it's a bit out of date now.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.
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