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Pros
Pro Written by industry leader
Burt Beckwith is an active member of the Grails community. He frequently speaks at conferences and answers Grails-related questions on stackoverflow.
Pro Exhaustive resource for advanced Grails
Cons
Con Not for beginners to Grail
This book is written for developers who already have some Grails experience. It's an in-depth, comprehensive guide to advanced Grails.