Which innovative book packaging format, introduced in 1935, revolutionized the publishing industry by making literature more affordable and accessible to the masses?

The publishing industry has seen numerous packaging innovations throughout history that have transformed how we interact with books. From the shift from scrolls to bound codices, to the introduction of paperbacks, to modern e-readers - packaging decisions have dramatically impacted literature's accessibility and popularity. This trivia question focuses on a revolutionary packaging innovation that democratized reading in the 20th century.

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Penguin Paperbacks
The Book-of-the-Month Club boxed set
Leather-bound Franklin Library editions
Random House dust jacket design