Nelson writes a breezy and somewhat flippant introduction to a socially conscious computer text. He argues that computers are a necessary part of modern life (modern being 1974, for a little perspective) and that everyone should be computer-literate, but that there are few opportunities for people who are not to become so, and to that end he has written this book to explain the basics.
Rather than coming from a solely technological perspective, though, Nelson is writing from a tradition of social radicalism. He talks about the "computer priesthood," the tendency of computer professionals to hoard knowledge, as unique not in its existence (doctors and engineers do the same thing) but in the impact of it. Since everyone has to deal with computers, the hoarding of knowledge is a serious problem. He castigates professionalism in general in a sidebar, preferring an informed citizenry to a decision-making professional class.
The goal of his book, then, is to create that informed citizenry. Nelson objects to popular objections to "the computer," saying instead that people should object to bad computer systems, not computers in general. He wants everyone to understand computers enough to understand the way people want to apply them in policy and in daily life, so that they can react from an informed position instead of a position of technophobia.