Today I published the completed "The Groovy 2 Tutorial" through LeanPub: https://leanpub.com/groovytutorial.
It's been a real labour of love as I researched the Groovy language, wrote the text and example code, edited and reviewed my work and even prepared a companion website (http://www.groovy-tutorial.org/). In all it's 443 pages and 64k words.
As Groovy is an open source effort I've made the full text available online under a CC-BY licence. You can also purchase a copy to receive the PDF/ePub version.