I received an email from Amazon today, with the following recommendation in it:
Dear Amazon.com Customer,
We've noticed that customers who have purchased or rated books by Aristotle have also purchased If You Got It, A Truck Brought It: Our First Adventure by Robin Smith. For this reason, you might like to know that If You Got It, A Truck Brought It: Our First Adventure is now available. You can order yours for just $8.99 by following the link below.
Notice the basis of the recommendation: because I'd purchased books by Aristotle, they recommended a particular children's book. I smell a statistical cluster in there somewhere.