The book reveals a wide variety of semantic and syntactic factors (some of them not very well known to linguists working on English) that control the usage of certain grammatical patterns. It also shows what kinds of sentences the linguist working on a nonnative language should check with native speakers of the language to prove or disprove his initial hypothesis. So in a third sense, Professor Kuno's study might be called a textbook of field methods in linguistic analysis.
Susumu Kuno is Professor of Linguistics, Emeritus, in the Department of Linguistics at Harvard University.