The trouble is worse when you buy presents for others- my "recommended" list on Amazon gets filled up with chick lit books similar to ones I've bought for Herself, and Bob the builder and Thomas the Tank Engine stuff after I've bought presents for my nephews.
Also if you've ever bought a "Best of" of "Rough guide to" anything, they think you'll like the whole series, and when you buy the latest by author X you get the back catalogue added (which I've usually got/read/ heard.)

RtS