This book is an excellent way to get in to Gradle. It starts with the basics and builds from there.
It is a practical guide to using Gradle with a lot of code examples that show you its possibilities.
Because of al these code examples it's also a great reference when you're working on a Gradle build.
In the books code samples you can see the genius of its author. He is well known in the Groovy and Grails community
for live coding presentations and for his mr HAKI blog which is busting at the seams with
practical Groovy, Grails and Gradle posts.
A book worth reading and well worth the investment!