Which pioneering machine learning feature revolutionized book discovery for readers when introduced by a major online retailer in the late 1990s?

The publishing and bookselling industry has undergone dramatic transformations in recent years. From small independent bookstores to massive retail chains and now to algorithm-driven online recommendations, the way literature reaches readers has evolved significantly. This trivia question explores how modern retail analytics and machine learning have changed how readers discover new books and authors in today's market.

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Collaborative filtering recommendation system that suggested books based on purchase patterns of similar customers
Sentiment analysis of book reviews to create emotional response profiles for readers
Predictive text completion that would auto-generate sequel ideas for popular books
Visual recognition software that matched book covers to readers' aesthetic preferences