What it used to be called:
Paperbacks found their way into pockets everywhere.
The Great Depression created a market for inexpensive literature, and the first major paperback publisher, Penguin, stepped in to serve it. (Its early bestsellers included titles by Agatha Christie and Ernest Hemingway.)
With the rise of paperbacks, readers and publishers needed to identify hardcover books as such, and that term appeared in 1949.
An early use:
"The author of a big smash will probably make somewhere between $40,000 and $100,000 on the hardcover item that begins the great chain of success." Life, July 31, 1964