So why should fewer books result in more sales? People can only handle so much choice. In a 2000 issue of Journal of Personality and Social Psychology a study showed that when shoppers are given the option of choosing among smaller and larger assortments of jam, they are more interested in the larger assortment. But when it comes time to pick just one, they're 10 times more likely to make a purchase if they choose among six rather than among 24 flavors of jam.
Another reason that I read online - they also looked at cereals. No one goes to the store and expects to find the cereal boxes with only the side panel facing out.
I guess we had all just better hope our books make the cut.