The iPhone became the world’s best-selling smartphone partly because Steve Jobs was obsessed with the ergonomics of everyday life. If you want people to carry a computer, it had to hit the “sweet spot” where it was big enough to display “detailed, legible graphics, but small enough to fit comfortably in the hand and pocket.”

Seventy-five years ago, another American innovator had the same epiphany: Robert Fair de Graff realized he could change the way people read by making books radically smaller. Back then, it was surprisingly hard for ordinary Americans to get good novels and nonfiction. The country only had about 500 bookstores, all clustered in the biggest 12 cities, and hardcovers cost $2.50 (about $40 in today’s currency).

De Graff revolutionized that market when he got backing from Simon & Schuster to launch Pocket Books in May 1939. A petite 4 by 6 inches and priced at a mere 25 cents, the Pocket Book changed everything about who could read and where. Suddenly people read all the time, much as we now peek at e-mail and Twitter on our phones. And by working with the often gangster-riddled magazine-distribution industry, De Graff sold books where they had never been available before—grocery stores, drugstores and airport terminals. Within two years he’d sold 17 million.

“They literally couldn’t keep up with demand,” says historian Kenneth C. Davis, who documented De Graff’s triumph in his book Two-Bit Culture. “They tapped into a huge reservoir of Americans who nobody realized wanted to read.”

Other publishers rushed into the business. And, like all forms of new media, pocket-size books panicked the elites. Sure, some books were quality literature, but the biggest sellers were mysteries, westerns, thinly veiled smut—a potential “flood of trash” that threatened to “debase farther the popular taste,” as the social critic Harvey Swados worried. But the tumult also gave birth to new and distinctly American literary genres, from Mickey Spillane’s gritty detective stories to Ray Bradbury’s cerebral science fiction.

The financial success of the paperback became its cultural downfall. Media conglomerates bought the upstart pocket-book firms and began hiking prices and chasing after quick-money best-sellers, including jokey fare like 101 Uses for a Dead Cat. And while paperbacks remain commonplace, they’re no longer dizzingly cheaper than hardcovers.

Instead, there’s a new reading format that’s shifting the terrain. Mini-tablets and e-readers not only fit in your pocket; they allow your entire library to fit in your pocket. And, as with De Graff’s invention, e-readers are producing new forms, prices and publishers.

The upshot, says Mike Shatzkin—CEO of the Idea Logical Company, a consultancy for publishers—is that “more reading is taking place,” as we tuck it into ever more stray moments. But he also worries that as e-book consumers shift more to multifunctional tablets, reading might take a back seat to other portable entertainment: more “Angry Birds,” less Jennifer Egan. Still, whatever the outcome, the true revolution in portable publishing began not with e-books but with De Graff, whose paperback made reading into an activity that travels everywhere.