I don't think I can point to only one resource but I would take a path (which you can customize based on your level of understanding of REST). I'm somebody who would like to get my concepts real clear first and then think about the tools to implement the concepts.
If you haven't read chapter 5 of Roy Fielding's dissertation, I'd start from there. It's an excellent piece of writing, and what better source to learn from than the master himself.
Infoq is an excellent source for REST articles, helped me significantly, here is a compilation of REST resources. Also on Infoq read this excellent article by Allamaraju.
If you are looking at books, the two that I liked so far: RESTful Web Services Cookbook and RESTful Web Services
As you are looking for Java-based services -- I'd get familiar with Jersey and/or RESTEasy. Write as many small programs as you can, what's the best way to get familiarized.
When you are at a point to try an exciting RESTful framework that's based on Hypermedia constraint I'd explore Restfulie.
Obviously, I haven't provided a single resource, but something in the lines I've provided would serve well, IMO.
