Which packaging design innovation in the 1930s revolutionized book marketing and accessibility, leading to the modern paperback revolution?

Book packaging design has evolved dramatically over centuries, from simple protective bindings to sophisticated marketing tools. Today's book covers are carefully crafted visual statements that communicate genre, tone, and content while enticing readers. This trivia question explores a significant historical innovation in book packaging that revolutionized the publishing industry and how readers interact with literature.

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Penguin Books' standardized color-coding system and minimalist covers
Dust jackets with elaborate foil stamping and embossing
Leather-bound limited editions with gilded page edges
Transparent plastic protective covers for library circulation