The Pragmatic Bookshelf | Programming Groovy
From the wikipedia article on Groovy:
Most Java code is also syntactically valid Groovy. […]
In July 2009, Strachan wrote on his blog that “I can honestly say if someone had shown me the Programming in Scala book by Martin Odersky, Lex Spoon & Bill Venners back in 2003 I’d probably have never created Groovy.”[1] Strachan left the project silently long before the Groovy 1.0 release in 2007.
Maybe I better focus on Scala than on Groovy.
Update 2011-07-01:
Purchased it as a used book through Amazon.de.