Smithsonian Magazine has a short post on the origin of the paperback book in the United States:

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.

Per Wikipedia, the first ten numbered Pocket Book titles were:

1. Lost Horizon by James Hilton
2. Wake Up and Live by Dorothea Brande
3. Five Great Tragedies by William Shakespeare
4. Topper by Thorne Smith
5. The Murder of Roger Ackroyd by Agatha Christie
6. Enough Rope by Dorothy Parker
7. Wuthering Heights by Emily Brontë
8. The Way of All Flesh by Samuel Butler
9. The Bridge of San Luis Rey by Thornton Wilder
10. Bambi by Felix Salten

An important note: the Pocket Books were the first paperback books in the U.S.  But it was Albatross Books, a German publishing house based in Hamburg. that produced the first modern mass market paperback books.

Albatross was founded in 1932 by John Holroyd-Reece, Max Wegner and Kurt Enoch. The name was chosen because “Albatross’ is the same word in many European languages. Based on the example of Tauchnitz, a Leipzig publishing firm that had been producing inexpensive and paperbound English-language reprints for a continental market, Albatross set about to streamline and modernize the paperback format.

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Related: How the paperback novel changed popular literature (also from Smithsonian Magazine)