E-book sales in the U.S. across all retailers in 2013, the last year that data were available, topped $3 billion, out of about $14.6 billion in total U.S. book publishing sales. With Amazon being the leader in e-books, it has a giant target on its back. Of late, Amazon has gotten so far ahead that the target may appear incredibly small -- and the arrows traditionally used to attack it flimsy. That may be changing. The five aforementioned publishers sell a majority of the books most people want to read. Until recently, many of them were signed to agreements with Amazon, Barnes & Noble ( BKS ) and others that allowed the retailers to set e-book prices. Today, four of the five are on the "agency pricing" system, meaning that they set their own prices and that prices across retailers are usually the same. This is hugely important for a new player like Oyster, which can now focus on competing with Amazon on the quality of its service and not on price.