Dan Fairs wrote an article on Web API design with a focus on efficiency issues for mobile devices. The article starts off with a very good and easy-to-follow description of what a REST API actually is, including examples of resources, representations, and interactions. He then presents some real-world practicalities, such as dealing with bad HTTP clients and returning rich error responses.
In terms of mobile devices, Dan notes that you want to minimize the number of necessary round trip API calls, which means including more information in a single response, i.e., introducing data redundancy or denormalization. Dan presents a technique that allows clients to specify how much redundant data is provided in a response based upon a parameter in the HTTP Accept header. This effectively allows different clients to tune their behavior and request the appropriate representation for the given application.