Building and Testing with Grade. This book is short with around 100 pages only. You need to background with other build tools like Ant and Maven to fully appreciate gradle. It is not a very exciting read, nor too boring. I'll rate 2.5/5 in terms of fun factor.
Ruby Quick Syntax Reference. Good short book to learn from. Read while checking internal documentation, ruby-doc.org web site, and an interpreter for best result. [***--]