The Definitive Guide to Grails 2

Very good book on Grails. It could go deeper on some topics.

I have been developing a complex web application in Grails for almost a year. This is the best reference I have found on Grails.

This is a good book to get to know grails with the condition you worked with a similar framework. If you worked with ruby on rails, this book will provide you a smooth start to grails.