<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 idea behind collaborative filtering is that it is easier to predict the future if you know what other people liked in the past. To do this, Amazon collects lots of data from people who have bought a book, including the author, the publisher, the genre, the price, and the star rating. Amazon then looks at this data to see what other people bought that are similar to you, and recommends those books.