Which book packaging innovation, first widely popularized in the 1930s, made literature more affordable and accessible to the masses through a smaller format and inexpensive production?

Throughout publishing history, certain innovations in book packaging and design have revolutionized how we interact with literature. From dust jackets to audiobooks, these innovations have shaped our reading experiences and helped authors reach wider audiences. This trivia question focuses on a groundbreaking packaging innovation that transformed the accessibility and popularity of literature in the 20th century.

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The mass-market paperback (pioneered by Penguin Books)
The leather-bound collector's edition with gilt edges
The hardcover book with removable dust jacket
The spiral-bound manuscript format