Which e-commerce innovation had the most significant impact on book discovery by introducing the first widely-used collaborative filtering system that told readers 'Customers who bought this item also bought...'?

The book industry has undergone dramatic transformations in how readers discover and purchase literature. From traditional bookstores to algorithm-driven recommendations, the retail experience for literature lovers continues to evolve. This trivia question explores how modern e-commerce platforms have changed the way we discover and consume literature. Test your knowledge about the intersection of technology and literary retail!

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Amazon's recommendation engine (1998)
Barnes & Noble's 'Customers' Picks' feature (2001)
Goodreads' social recommendation platform (2006)
BookCrossing's shared library system (2001)