It was fairly recent:
Here’s a little perspective: In 1939, gas cost 10 cents a gallon at the pump. A movie ticket set you back 20 cents. John Steinbeck’s The Grapes of Wrath, the year’s bestselling hardcover book, was $2.75. For a nation suffering 20 percent unemployment, books were an impossible expense.
Robert de Graff's "Pocket Books" changed all that:
[De Graff] knew that printing costs were high because volumes were low—an average hardcover print run of 10,000 might cost 40 cents per copy. With only 500 bookstores in the U.S., most located in major cities, low demand was baked into the equation. … So de Graff devised a plan to get his books into places where books weren’t traditionally sold. His twist? Using magazine distributors to place Pocket Books in newsstands, subway stations, drugstores, and other outlets to reach the underserved suburban and rural populace.
It worked: after a week, Pocket Books sold out its initial 100,000 copy run – and democratized reading forever.