I would like to know if this book requires prior Groovy knowledge? I am a relatively seasoned Java Dev, with only the vaguest notions of Groovy. Will this book bring me up to speed on Groovy, or at least all the bits of Groovy that Grails uses? Or should I read a Groovy book before hand? And if so any suggestions? Groovy in Action seems to be the market leader.
The book mainly targets Java developers, so no, you don't need to know Groovy beforehand. Chapter 2 is a primer dedicated to Groovy that will get you up to speed with what you need to use Grails effectively. If you fall in love with Groovy and use it extensively, then it's worth getting Groovy in Action as well. It's probably worth getting even if your Groovy work is limited to Grails.
The chapter does introduce closures to a level that should be more than sufficient for most Grails work. For more advanced work, particularly with plugins, then it's worth getting a book dedicated to Groovy that covers delegates, name resolution, and other closure features.
Try "Programming Groovy" by Venkat Subramaniam for an excellent introduction to how Groovy works (as well as some pretty clever stuff on dynamic metaprogramming). Look out especially for the sections on closures and builders, which will really help you with understanding Grails as well.
nicole, this book is very good and I've just choose it as my first grails book, the sample is very interesting and meaningful, however you need open the grails online guide (http://grails.org/doc/latest/) as some tags' usages are not up to date.