Which innovative book packaging technique was pioneered by Penguin Books in the 1930s and revolutionized the mass-market paperback industry?

In the world of arts and literature, packaging design plays a crucial role in attracting readers and collectors. Book covers, special editions, and collector's box sets can significantly influence consumer decisions in bookstores and online marketplaces. This trivia tests your knowledge about innovative packaging trends in the literary market that have revolutionized how we interact with physical books in an increasingly digital age.

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Color-coded book spines by genre (orange for fiction, green for crime, etc.)
Die-cut covers with windows revealing interior illustrations
Thermochromic ink that changes color when touched
Embossed leather slipcases for all standard editions