Which revolutionary packaging innovation in the publishing industry during the 1930s-40s made books more affordable and accessible to the average American?

Book packaging has evolved dramatically over the centuries, from handcrafted leather bindings to today's interactive digital experiences. Publishers continuously innovate with cover designs, special editions, and packaging formats to attract readers in an increasingly competitive market. This trivia question explores a significant packaging innovation that revolutionized the accessibility and popularity of literature for the mass market.

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Paperback books (specifically Penguin and Pocket Books formats)
Cloth-bound hardcover with dust jackets
Leather-bound subscription series
Spiral-bound illustrated editions