Which innovative book cover design approach, introduced in the 1980s, allowed readers to personally interact with the cover art and is credited with increasing sales by over 30% for titles using this feature?

The publishing industry has seen numerous packaging innovations designed to attract readers and boost sales. Sometimes these creative choices have stirred major controversy or transformed the market. This trivia question explores a famous packaging innovation that changed how readers engage with book covers. Test your knowledge of literary product design history!

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Holographic covers that changed images when viewed from different angles
Die-cut windows revealing partial images from internal illustrations
Scratch-and-sniff covers that released scents related to the book's content
Heat-sensitive covers that revealed hidden images when touched