McLuhan writes sentences that appeal to the spiritual crowd but that make
little rational sense and are mostly unsupported by empirical evidence.
Fundamentally, the book argues that the medium is the message because it shapes how humans communicate and act. Technological media are so important that they should be considered like natural resources.
After these initial pages, the book goes downhill. Confusingly, McLuhan distinguishes cool media and hot media. Hot media are "high definition", emphasize one sense over the others, and are low in participation: the waltz, phonetic alphabet, radio, cinema, photography. Cool media emphasize participation: formal dance, ideogrammatic writing, telephone. Television is cool, or low definition, because it requires an effort on the part of the viewer. Hot media in hot cultures are entertainment. Hot media in low cultures are as upsetting as cool media (such as television) is for hot cultures like ours. The difference is never fully explained and is empirically dubious.
The history of each medium exhibits what Kenneth Boulding called a "break boundary" at which the system suddenly undergoes a sort of phase transition. But McLuhan does not elaborate.
He repeats a concept that was prominent in "Gutenberg Galaxy": because a medium changes the way the body/brain works, a reorganization of the relative importance of the senses occurs; when one sense is emphasized, others are penalized.
Writing (the phonetic alphabet and then its logical consequence, the printing press) is an extension of the sense of sight that caused the fragmentation typical of the Western analytical approach. Number, on the other hand, is an extension of the sense of touch. Mechanical technologies for extending our physical bodies put us out of touch with ourselves and created a society of individual points of view. The printing press, that translates knowldege into mechanical production, turned any kind of space into an ordered flat space. The printing press therefore created the sense of infinite that created calculus (study of infinitesimals). Print technology basically trasformed the positional zero of the Middle Ages into the vanishing point of the Renaissance into the concept of infinite and infinitesimal.
The book is much better if you just read the sentences than if you try to make too much sense of them:
McLuhan rehashes his favorite culprit, the phonetic alphabet, that made semantically meaningless letters correspond to semantically meaningless sounds, thus setting Western civilization on a course of specialization and industrialization (uniform sequential processes) especially after the printing press amplified this phenomenon. Languages that use nonphonetic scripts, such as Chinese, retain a rich store of experience, each ideogram retaining "a total intuition of being and reason"
The book loses credibility very rapidly once it starts analyzing the various technologies that interest McLuhan. Chapters such as the one on clothing are full of strong opinions with little or no empirical evidence to support them. He often sounds more like Freud than like a cultural anthropologist. No wonder that the chapter on Money begins with a sentence on Freud's concept of (quote) "character and anal eroticism". McLuhan writes sentences that make you think, such as "Photography created a world of accelerated transience". But he offers little proof for such statements.
Inspired by Bergson, McLuhan thinks that language is a technology that has caused some kind of psychological split in the collective unconscious that used to rule (but McLuhan quotes Bergson from a book, "Creative Evolution", that was vastly discredited about the biological synthesis of the 1920s). Language, basically, removed a fundamental element of what it means to be human. McLuhan thinks that electrical media can restore that collective subconscious. He phrased it better in a Playboy interview: "Electricity makes possible an amplification of human consciousness on a world scale and without any verbalization at all". Then he goes very spiritual saying that this will lead to "a perpetuity of harmony and peace".
McLuhan thinks that "Western man is being dewesternized", as the electronic media are creating a village, except that this is a "global" village.
Read instead the much better "The Gutenberg Galaxy" (1964).
P.S. McLuhan never wrote the widely quoted sentence "We become what we behold. We shape our tools and then our tools shape us" or "First we build the toolds, then they build us". It was written by John Culkin.