I think the message of your book is spot on. My pitch line for Groovy is I tell folks that it is the most productive language a person who knows Java can use. I also think it has great potential for teaching programming and naturally I used it for the solutions I showed students in the class I just finished teaching on Computational Linguistics Fundamentals (http://courses.washington.edu/ling473/).
A Groovy toolkit I think Java programmers can benefit from is Ratpack (http://www.ratpack-framework.org). I've used it (both the original version and the new rewrite) to do all the quickie UIs I need for exploring the data sets and output from programs I develop. While Grails is great, a lightweight app using MarkupBuilder-based pages and forms is all I usually need (since I rarely work with relational databases). Like using Spock for testing and Gradle for building, using Ratpack is a highly productive fluent(ish) way to make Java code do UI stuff.