Which publishing innovation introduced 'Heat-Sensitive' book covers that reveal hidden artwork or text when touched?

In the world of book publishing, packaging design plays a crucial role in attracting readers. Publishers constantly innovate their cover designs, special editions, and packaging to appeal to different market segments. This poll tests your knowledge about a revolutionary packaging design that changed how readers interact with literary classics. Think about memorable book packaging innovations from the past decade.

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Penguin's 'Touch Classics' series featuring Jane Austen novels
Visual Editions' 'Sadly, By Accident' collection
Fahrenheit 451 special edition by Ray Bradbury (designed to be read only when heated)
HarperCollins' 'Sensation Series' thriller novels