I enjoyed the Amundsenn book mentioned by irene_t and the collection+json specification, but found that my API consumers ignored the links in the collection+json response in favor of hardcoding URLs in their consuming apps.

Although my swing at RESTful API development wasn't particularly successful I do like the book. It helped me think more about the HTTP protocol and the well-defined nature of HTTP response codes.

I HIGHLY recommend the Clojure liberator library with its decision-graph model - it made my API development so much easier to reason about and maintain with respect to the HTTP response codes. I actually found just looking at the decision graph to clarify some of my thoughts and questions on HTTP-based API design.