Which revolutionary book packaging innovation, introduced in the 1930s, was originally marketed as 'Books for the Masses' and fundamentally changed reading accessibility?

The publishing industry has undergone remarkable transformations in how books are packaged and presented to consumers. From dust jackets to special editions, the physical presentation of literature has evolved with changing consumer preferences and technological capabilities. This trivia question explores a significant packaging innovation that revolutionized how people interacted with books in the 20th century.

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Penguin's color-coded paperback system (orange for fiction, green for crime, etc.)
Random House's dust jacket with transparent acetate covering
Simon & Schuster's 'Pocket Books' with rounded corners
Doubleday's cloth binding with gold leaf stamping