The Gutenberg Galaxy is a 1962 book by Marshall McLuhan. It attempts to position our time as a period of transition between the "literary period," which overcame the older "oral" period in the centuries immediately following the invention of printing, and the "electronic," which McLuhan describes variously as following either Marconi's invention of the telegraph or James Clerk Maxwell's discoveries of electromagnetism. The book is a sort of extended eulogy for this literary period (though McLuhan comes not to praise the Gutenberg Age, but to bury it).
Since this is typical McLuhan fare, the "oral" period is described as characterized by "hot" media and auditory/tactile perception and the "literary" by "cool" media (print) and visual perception. The electronic age is supposed to dialectically revive the oral period and move away from the evils of the Gutenberg Age. This set of metaphors, particularly the auditory-visual opposition, is the overriding message of the book.
The literary period, McLuhan says, is characterized by several interrelated drives: homogeneity (the two-dimensional geometric perception that McLuhan attributes to the literary period is unable to handle heterogeneity); jobs replacing roles; nationalism replacing tribal identity; the aforementioned visual geometric bias; and so on--essentially everything commonly considered to be part of modernity. There are many lengthy theoretical excursions from these basic ideas, some of which are even interesting, but that's the foundation.
As I mentioned above, the book is standard McLuhan fare. This means that it reads like a cross between a freshman term paper in English Literature and a Lyndon LaRouche pamphlet exposing the British-Nazi-Jewish influence on the Department of the Interior. From the first, it gets freaky-but-marginally-plausible readings of Joyce, Shakespeare and a host of other authors; from the second, it gets a mind-bogglingly expansive database of citations and degree of creative interpretation thereof. That having been said, much of it is profound, and the book is worth it just for some of the quotes (the passage describing the reaction of African tribesmen to a film on sanitation, and the snippet from the New York Times describing a new phonetic alphabet, for instance). McLuhan's vision of the electronic period, which has been just around the corner for a century and a half now, bears an interesting resemblance to postmodernism or postmodernity.
The book's most important insight is into the differences between oral and literary cultures. I recommend, however, that the reader mentally replace all instances of "pre-literate" with "non-literate": McLuhan has indulged in a bit of amateur historiography and decided that all peoples must eventually follow the Great Yellowbrick Road of Literacy, completely ignoring, for example, the Inca--who managed a vast empire without a literate culture--and his own insights in War and Peace in the Global Village to the effect that oral cultures may well reach the electronic period directly.