Though the notion of a scientific textbook has been around for almost three centuries, the category has hardly been stable. The plasticity of the textbook genre may be illustrated by recent variations as well as long-term trends. In this brief essay I examine two idiosyncratic but highly successful physics books, each published in the mid 1970s, whose production, marketing, and adoption reveal some of the slippage between such categories as textbook, scholarly monograph, and popular best seller.