Which marketing technique was pioneered by the 1997 campaign for 'Harry Potter and the Philosopher's Stone' that is now standard practice in publishing?

Famous marketing campaigns have transformed obscure books into cultural phenomena. The publishing industry has seen remarkable successes through innovative promotional strategies that combined traditional media with emerging platforms. Test your knowledge about one of the most influential book marketing campaigns that revolutionized how literary works are promoted and sold to the public, creating unprecedented buzz and reader engagement.

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Separate adult and children's cover designs for the same book to target multiple demographics simultaneously
Releasing the first three chapters free online as a digital sampler before the print release
Creating interactive augmented reality experiences tied to specific book locations
Publishing the book first as a limited collector's edition before the mass market version