This book came out last year, I believe coinciding with SDCC. Anyway, it was published by Sideshow Toys and it is a collection of customized artwork created around the 1/6 scale R2 units sold by Sideshow. Many artists took a crack at it, and there are some great ones in there.
The book is still available at Sideshow Toys for $20.
You can also look through the awesome gallery using this link.