One of the best introductory papers to prototyping was written some time ago by Stephanie Houde and Charlie Hill when they were both working with Apple's Human Interface Group; and it deserves to be more widely known. This is one of those seminal papers that critically informed Plot's way of seeing, thinking, and doing. It provides both a strong rationale for, and outlines the practical constraints around, the prototyping approach. In it they ask a simple question — What do prototypes prototype?
In answer, they describe the different dimensions of design issues that a prototype can address. And even though some of the references to prototyping tools may seem dated (Hypercard anyone?), the issues remain salient and clear for anyone using prototypes as a means to communicate with and through.