Which innovative product design feature was first introduced with J.K. Rowling's Harry Potter books that later became standard practice for creating 'collector appeal' in publishing?

The publishing industry has evolved significantly over the decades, with innovative marketing techniques helping certain books achieve extraordinary success. Behind many bestsellers are creative product design and advertising strategies that transformed ordinary books into cultural phenomena. Test your knowledge about some of the most successful marketing innovations in literary history!

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House-specific cover variants (Gryffindor, Slytherin, etc.)
Adult and children's cover designs released simultaneously
Interactive augmented reality book covers
Glow-in-the-dark cover illustrations