[What] is a "classic" Heinlein work? Most criticism of Heinlein begins and ends here. Invariably, each individual critic has chosen the works he likes best, dubbed them classics, and consigned the rest to oblivion…. The years to be covered in this study include, basically, the 1940s and 1950s—the period of the stories and novellas, and the novels of juvenile adventure. Unfortunately, there is no touchstone which allows a reader infallibly to pick "classics" out of this span. What is possible, however, is a definition of process that will permit us to study Heinlein's evolution as a writer over two long and full decades. (p. 3)

[If] chronological periods are marked off at all [in Heinlein's work], they must be ordered in terms of genre. The use of a given form, in Heinlein's case, was dictated in large part during [his] early and middle years by the vagaries of...