Pinker begins with a quick survey of the most obvious evidence for the innateness of language — the growth of creoles from pidgins, the existence of sign languages, the even distribution of language ability, and studies of brain-damaged speakers. He also considers the relationship between language and thought, arguing against claims for a strong dependence of the latter on language. This is followed by three chapters on the core of linguistics — one on generative grammar, one on matters lexical and morphological, and one on phonetics and phonemics. Another chapter describes attempts to automate aspects of language (such as parsing and speech recognition) and the unexpected difficulties encountered.
If human language is innate, then why is there such a variety of languages? Pinker devotes a chapter to exploring the ways in which languages vary, the ways in which they change with time, and some of the attempts at reconstruction of human linguistic history (including a reasonably even-handed appraisal of Greenbergian lumping). Separate chapters are devoted to language acquisition by infants, to the biological (genetic and ontogenetic) underpinnings of language, and to the evolution of language. Here Pinker disagrees with Chomsky, seeing no problems with a selective explanation for the evolution of language. The final chapter touches on other aspects of the human mind which seem likely candidates for innate "modules" and examines their relationship to linguistic competence.
Pinker has a great sense of humour and his tone is almost chatty, with illustrative examples taken from newspaper headlines, Yiddish jokes, the Watergate tapes, literary texts, and an assortment of other places. But his anecdotal bent never obscures his argument and The Language Instinct provides clear explanations of a number of quite difficult topics. Though it is aimed at (and is certainly accessible to) a general audience, it could also be used as an introductory textbook. It is simply the best popular work on linguistics around: if you are only going to read one book on the subject then Pinker's should be the one.