I have too many books. I used to think cataloging them would help, by keeping me from traveling half an hour to a library to get a volume I had on my shelves. I tried out LibraryThing, a hybrid of catalog and social bookmarking, when it was cheap and small, but realized the cataloging process would take way too long. Sadly, I didn't join.
Now, with more than ten million books cataloged by its members, LT offers a feature called the Unsuggester, available to all. You name a book, and the unsearch engine spits out the titles that are least likely to appear in members' catalogs along with it. It's not exactly, "If you like X, you'll hate Y," but it's close. I tried a few:
A Textual Commentary on the Greek New Testament is least compatible with Chicago author Audrey Niffeneger's excellent SF The Time Traveler's Wife.
The Chicago Manual of Style is least compatible with Shopaholic Takes Manhattan.
William Cronon's Chicago classic Nature's Metropolis is least compatible with The Devil Wears Prada.
Richard Dawkins's controversial The God Delusion is least compatible with the young-adult Girls in Pants: The Third Summer of the Sisterhood.
Daniel Dennett's Breaking the Spell is least compatible with The Divine Secrets of the Ya-Ya Sisterhood. And Sam Harris's The End of Faith is least compatible with Alanna: The First Adventure.
I'm seeing a pattern here, but it's not what I expected.