While Dan Saffer's forthcoming book Interactive Gestures: Designing Gestural Interfaces is not out yet, you can download and read the first chapter. Aptly titled "Introducing Interactive Gestures", Saffer covers the recent and formative histories of direct manipulation interfaces, as well as key definitions, and relevant usability/design issues in an approachable manner.
The chapter is readable, while still providing appropriate references to human factors principles and technologies. If you're new to the world of gestural interfaces, this is a great place to get an overview of the field. If you're already knowledgeable, this is a useful refresher, and you might learn some new terms like "iceberg tips" (touch points that are larger than they visually present).
I did spot one point of dispute. In his explanation of affordances (p. 30), Saffer refers to James Gibson as a "cognitive psychologist". While Gibson was a psychologist, his theories of perception were actually contrary to the cognitive movement - Gibson posited a theory of direct perception where information is perceived without the need for any intermediating mental interpretation (i.e., cognition). A more accurate label would have been "ecological psychologist" - but since that's not a school of perception known by most people, simply "psychologist" would probably be best.
Forgive me for being academic.