Books
Apart from the online resources documenting Groovy and Grails, Graeme Rocher and Guillaume Laforge have also writen books on those topics. G2One Inc. recommends these books for a better and thorough understanding of the language and framework.
The Definitive Guide to Grails
Writen by Grails lead and CTO of G2One Graeme Rocher,
The Definitive Guide to Grails presents the Grails web framework in depth.
You will discover all the layers of the framework,
from the GORM persistence layer, to the advanced AJAX tag libraries,
while still showing you how to best test your applications along the way.
Groovy in Action
Guillaume Laforge, Groovy project manager and VP Technology of G2One,
has collaborated along with Dierk Koenig to deliver the Groovy in Action best-seller published by Manning.
While part 1 details the Groovy language itself through all its constructs and semantics,
part 2 focuses on all the advanced libraries provided with Groovy,
whereas part 3 will give you all the handy real life examples you need everyday.