Which innovative packaging technique first popularized in the 1980s-90s revolutionized young adult mystery series marketing by allowing readers to 'solve' the mystery alongside the protagonist?

In the competitive world of publishing, book packaging has evolved dramatically to capture reader attention. From embossed covers to interactive elements, publishers continuously innovate to make physical books stand out in an increasingly digital marketplace. This trivia question explores a significant packaging innovation that revolutionized a particular literary genre's marketing approach.

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Choose Your Own Adventure format with multiple endings
Sealed 'evidence packets' with physical clues included in the book binding
Invisible ink reveals that could only be read with special decoders
Augmented reality covers that animated when viewed through special glasses