The first part of the book covers the basics of the Groovy language syntax, including data types (such as GStrings), control structures and closures. This also includes a rather technical (but useful) section on class loading in the JVM and explains how Groovy augments standard Java classes effectively on-the-fly. The second part focuses on various parts of the Groovy libraries, and the third part covers a selection of advanced topics. The chapter on Scriptom integration in Windows is particularly interesting; the final chapter, on the Grails web framework, is written in a rather irritating and contrived conversational style.
Unlike the Scala book I read recently, whose purpose was to be merely factual, this book offers opinions to promote the Groovy language. Mostly this is reasonable although from time to time it becomes a bit grating, especially when subsequent paragraphs are somewhat handwaving on areas like performance. Several language features, such as closures, are now available in Java 8 so it remains to be seem whether these alternative languages have gained a sufficient foothold.