Which innovative product packaging format revolutionized the book industry in the 1930s, making literature more accessible and affordable to the masses?

The book publishing industry has seen numerous innovations in product packaging over the decades, transforming how readers interact with literature. From dust jackets to interactive elements, packaging has evolved to enhance both protection and marketing appeal. This trivia question explores a significant packaging innovation that changed how certain literary works were consumed and marketed to the public.

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Penguin's color-coded paperbacks with standardized design
Leather-bound subscription book clubs with gold embossing
Hardcover books with detachable bookmarks and reading guides
Limited edition boxed sets with author signatures