I'm not entirely sure how it impacts authors but I had always assumed books worked like most other products. The retail giants buy more of an item than the smaller guys can which allows them to get it for a cheaper price per unit which in turn lets them sell them cheaper without hurting their profit margins too much. Amazon isn't the only one though. Take Oathbringer for example. It's currently $17.39 on Amazon. It's the same price on Barnes and Nobles website and is actually significantly cheaper on Walmarts. Amazon also price matches frequently so sometimes when you see a cheap price it's actually from another retailer and Amazon is just trying to compete.