I just got through looking up a couple of books on Amazon for the links, and they're not particularly representative of my purchases. Amazon will duly record them though, and I'm sure I'll see no end of strange book recommendations for a while. A couple of them are even on a list that some lefty compiled to help in getting an FBI file.
That reminds me of an article in the dead-tree Wall Street Journal with a headline I stole above, which discussed how the behavior of Tivo led to some amusing situations. If you're not familiar with Tivo, it is a disk based video recording system which not only records TV content, but makes recommendations for future viewing based on past selections.
The recommendations feature is what leads to the weirdness. As I recall, the article suggested that one episode of "Queer as Folk" was enough to stamp a pink triangle on you. Some people cited in the article were trying to pick countervailing programs to get Tivo to change its behavior. Apparently this concept has even made it to certain sitcoms.
Like with Amazon, Tivo can be "trained". This involves "thumbs up" and "thumbs down" keys on the remote, and you can press them more than once for a given selection. After enough of this presumably you'll get better recommendations, and you won't have to worry about your Tivo talking behind your back.