Which publishing innovation did Bloomsbury introduce with the original Harry Potter books that revolutionized children's book marketing?

In the world of publishing and literature, effective branding can transform a book from a simple product into a cultural phenomenon. This poll explores one of the most successful product branding stories in modern literary history - a case that revolutionized how books are marketed to young readers while bridging the gap between children's and adult literature markets. Test your knowledge of this publishing industry milestone!

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Adult and child cover variants for the same book, allowing adults to read children's literature without stigma
First major series to use holographic cover technology to attract young readers
Pioneer of the 'digital companion' model with free online content accessed through book codes
Introduction of character-based merchandise bundles where toys were included with premium editions