An interesting idea:

  1. Take a single book from Amazon as a root.
  2. Take the recommended books for that book.
  3. Repeat until you have reached a distance of 20 from the original book.
  4. Add up the number of recommendations for each book.
  5. Calculate the BookRank from the Distance, Recommendations and SalesRank
  6. Display the 250 books with the highest BookRank.

Some examples are available on their site. (via findability)