In reply to Scott Preece.

Using what I'm interested in to make useful suggestions to me is fine. Using what I'm interested in as an indicator of what local tastes are (such as for the purpose of curating the books on the shelves) is also fine.

At some point "big data" uses do seem to cross a line, but the creepiness factor isn't really what bugs me the most. It's the idea that millions of people are performing unpaid labor that a huge corporation uses to make big bucks that bugs me. Dozens of people performing unpaid labor that a small, local corporation uses to pick up a few extra dollars here and there seems different.