Which revolutionary brand strategy did Penguin Books pioneer in 1935 that transformed the publishing industry?

In the publishing world, branding has played a crucial role in how readers discover and connect with books. One publisher revolutionized the industry with their innovative brand strategy in the mid-20th century, making literature more accessible and recognizable to the mass market. Test your knowledge about this iconic publishing brand strategy that changed how we consume literature.

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Color-coded book covers based on genre (orange for fiction, green for crime, etc.)
First publisher to use celebrity authors' faces on all book covers
Introduction of the hardcover-first, paperback-later release strategy
First to bundle books with complementary music records