Challenge your prejudices some more: Read a book that you have panned or derided without actually having read it.
Some frequently panned books that come to mind include novels by Dan Brown, Stephanie Meyer, Nicholas Sparks and Danielle Steel. Others include such famous and/or infamous works of the more distant past, like philosophical and religious writings of all ages and eras, Marx’s Communist Manifesto, Joyce’s Finnegan’s Wake, and anything by the Marquis de Sade. Of course you should choose one that you have prejudices about.
Whether your prejudices are rebuffed or confirmed, you will at least now be able to pan or praise the book in question without hypocrisy.