Stating the obvious
Did you like The Confessions of St Augustine? Well, then you'll probably hate Sherrilyn Kenyon's Night Pleasures. Is Immanuel Kant's The Critique of Pure Reason on your bookshelf? In that case, stay clear of Confessions of a Shopaholic!
The Unsuggester safeguards you from book you may be foolish enough to think you'd like... but won't.
Of course, its not by any means foolproof! I happen to think a person might enjoy Screw the Roses, Send Me the Thorns and still find Little Women a good read, too!
And yes, before you ask. Yes.