"The Definitive Guide To Grails" (APress) by Graeme Rocher and Jeff Brown is the most comprehensive book on Grails that I've seen, but it's a reference book rather than a tutorial.
"Grails In Action" by Glen Smith and Peter Ledbrook (Manning) is an excellent and pretty thorough introduction to Grails based around a tutorial with lots of intelligent material on how and why things work a particular way. Plus their sense of humour is good fun.
And definitely check out Scott Davis's series of
Mastering Grails tutorials on IBM developerWorks.
Depending on your previous experience, I'd also recommend getting a good book on Groovy, because you'll find the language is extremely powerful if you start to appreciate the way it works, and especially the ways in which it is different from
Java. Grails uses a lot of clever Grooviness, on top of lots of clever Spring stuff, and it helps to understand at least in principle how it all works.
My favourite Groovy book is "Groovy in Action" by Dirk Koenig (Manning), which will give you an excellent introduction to the clever things you can do with Groovy. There's supposed to be a new edition out soon (next year?), but the 2007 edition is fine.
Venkat Subramaniam's book "Programming Groovy" (Pragmatic Programmers) is also good, but doesn't cover as much as the Koenig book.
Have fun!
Chris