A must have for any serious Groovy developers.
Groovy in Action (Second Edition) is, at the same time, a detailed overview of the language and core libraries characteristics and an in-depth description on how it works.
First part is dedicated to the language, with the usual list of syntax elements descriptions (operators, data structures, control structures..), including Groovy unique features like being together a dynamic and a static typing language or supporting both object oriented and functional programming styles, not mentioning the scripting capabilities.
Second part is devoted to the Groovy core library: the Groovy Design Kit, how to work with databases and web-services and how to handle JSON and XML.
Final part is dedicated to unit testing, concurrency and, of course, the domain specific languages, one of the traditional Groovy applications area.
I have found very interesting chapter 16 on how integrate Groovy in a Java application, using, for example, Groovy as dynamic business rules engine, and chapter 20 on the Groovy ecosystem (Gradle, Grails..) introduction.
Authors show not only a very strong knowledge of the language and its ecosystem but also an understanding of how Groovy fits in the real world applications. Very interesting and useful the adoption of the assertions statements to better explain the code examples.
This book, with 900+ pages, is not targeting occasional Groovy users but I think it is a must have (as introduction at the begin, as reference later) for anybody intended to seriously use Groovy.