Carolyn Snyder's Paper Prototyping: The Fast and Easy Way to Design and Refine User Interfaces provides the only complete guide to paper prototyping. It teaches you everything you need to know to successfully do paper prototyping and offers many practical tips. However, only about a third of the book is actually about doing paper prototyping. The majority of the book's content comprises a basic reference on usability testing. While some of the information on usability testing describes how to test paper prototypes, most of it is applicable to any type of usability testing. If you're already an expert in usability testing, you may not find this information as useful, but Snyder has honed her approach to usability testing over her many years of experience as a usability professional and provides a wealth of practical information.