Which innovative marketing strategy by Penguin Books in the 1930s revolutionized the publishing industry and made quality literature more accessible to the general public?

The publishing industry has seen remarkable marketing innovations that transformed how we discover and consume literature. From iconic brand identities to revolutionary retail strategies, publishing houses have shaped literary culture through clever promotion. This trivia question explores a pivotal moment in publishing history that democratized access to quality literature and created one of the most recognizable brand identities in the literary world.

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Introducing color-coded book covers based on genre (orange for fiction, green for crime, etc.)
Creating the first literary subscription box service with curated monthly selections
Pioneering the 'buy one, get one free' promotion for hardcover books
Establishing the first chain of dedicated bookstores with in-store cafés