<human>:  Why do they recommend books that I have already read?
<bot>: It’s because Amazon uses a technique called collaborative filtering to recommend books. The technique is based on the idea that if you like a book, then other people who liked the same book will probably like your next book. For example, if you like the book “Harry Potter and the Sorcerer’s Stone,” then other people who liked the same book might also like the book “Harry Potter and the Chamber of Secrets.”