While no one likes strict Plan-O-Grams for displaying books, it is common sense that consumers looking for one type of book would like to easily locate the other books in the same category. Therefore if you categorize and display similar books, you increase the chances for multiple purchases from one consumer.
For example, at any one point you likely carry multiple books that would be considered “Chick-Lit”... if they’re not on the Bestseller list, display them together and test how they do in your store. Not only does your customer know the selection you do carry, they can choose between titles, or choose to buy a few.