That is a very good question.
Everything in Groovy in Action is still 100% relevant.
There have been a lot of improvements to Groovy since the book came out but the concepts have not changed and the developers have been very careful not to break old code. This will remain true for all Groovy versions labeled 1.x.
It is still a long way to go until Groovy 2.0 (probably a year or so but don't take my
word for it). Groovy 2.0 will have new concepts and is likely to break 1.x code. My intention is to have a second edition for Gina for Groovy 2.0 but not before.
cheers
Dierk