If you look at Wikipedia’s list of the bestselling fiction writers of all time, you’ll notice that the majority are English-language authors of the 20th century. So Barbara Cartland’s on there, Agatha Christie has already equalled Shakspeare, Stephen King, Michael Crichton—they’re all on the list. This is because book publishing became, truly, industrialized in the 20th century (much like war). This happened particularly in the English-speaking world. (Latin America, China, India—none of these parts of the world have anything resembling the great New York publishing houses in their heyday.)
This phenomenon had immense effects, but the most insidious one has been on how people, both writers and readers, think of literature itself. It is the origin, for example, of the modern-day psychological division between “literary” (read: contemporary realist) novels and “genre” novels (whether science fiction, fantasy, or “historical fiction”). They have their own communities of readers, their own awards, their own luminaries (with occasional cross-over, like Margaret Atwood or Ray Bradbury). This division arose from the need of a massive book publishing industry to classify and label their products. Critics as recently as the 19th century, and in much of the 20th, don’t recognize this literary/genre dichotomy, which is so ubiquitous today among readers and writers.
The key here is that the distinction is, actually, not absolute. Cormac McCarthy’s The Road gets the classified, primarily, as a literary novel. To have one’s genre novel called a literary novel is a mark of prestige. The opposite is not the case; Margaret Atwood is known to dislike being called an author of science fiction. (She prefers we call it “speculative fiction.”)
This implicit hierarchy marks a second, massive shift in how we think–not just about literature, but about life. We seem to privilege the commonplace more than our predecessors; I can identify this love of the everyday, this conviction of the luminosity of the mundane, only in the Dutch masters who painted still lifes and domestic scenes. Literarily, we tend not to produce Michelangelos; we produce Vermeers.
This implicit hierarchy, let us not forget, used to be the exact reverse. The introduction of some supernatural or fantastic element—vampires, elves, other worlds, that kind of thing—shifts a book into the “genre” category. Supernatural and fantastic elements are widespread in Western literature from Homer on—it is supremely “literary,” in every sense, especially that of the “high” literary. You can ignore the Gods and Goddesses; Achilles’s horses turn around and talk to him. Odysseus and Aeneas visit the underworld and talk to ghosts. Beowulf slays the medieval equivalent of a swamp thing. Shakespeare’s most serious tragedies contain witches and ghosts. Goethe’s Faust has a whole Walpurgisnacht. I could go on. Contemporary realist elements have been, in more than one era, considered base, “low,” unworthy of True Literature—this is why producers and some editors used to cut Shakespeare’s earthier clown scenes (consider the etymology of “bowdlerized”). When Victor Hugo’s 1830 play Hernani mentioned a mouchoir (handkerchief) onstage, it caused a scandal. Today it’s precisely the handkerchief-mentioners, the contemporary-society realists, who are thought the most “literary” of writers.