20 years ago in Alaska there were several local competing grocery stores. Carrs (Safeway) came in and undercut everybody's prices, including selling many staple groceries at a loss. Everyone loved the low prices.
But the "win" for consumers was short-lived. You see, the local stores couldn't compete with selling at a loss, and so they eventually all went out of business. You think Carrs kept selling at a loss? Hell no, Carrs jacked up the prices above where the now-out-of-business local stores were selling in the first place. Carrs had no competition.
Amazon is essentially doing exactly this. Did you know Amazon is not a profitable company? They sell many things at a loss, more than just books. And just like the local grocery stores in Alaska, Amazon's competitors are feeling the pain. Where is Borders? How is Best Buy doing?
Do you think Amazon will simply sell at a loss forever? What do you think they will do when they've eliminated their remainder of their struggling competitors?
BTW - The publishers aren't happy either, despite getting paid the full price. Selling at low prices for a long time makes consumers accustomed to those prices. From their point of view, books are being devalued.
Predatory pricing is great for consumers... temporarily.