Programming Groovy: Dynamic Productivity for the Java Developer (Pragmatic Programmers)	Superb coverage of groovy meta programing
	Groovy Recipes: Greasing the Wheels of Java (Pragmatic Programmers)	Superb collection groovy code snippets
	The Definitive Guide to Grails	The original and the best but a little dated now.
	Groovy in Action	Good overall coverage