Amazon is most definitely not a monopoly. However, before Apple convinced the publishers to move to an Agency pricing model for ebooks, Amazon was definitely in danger of becoming
a monopsony in the ebook space.
Amazon's entire business model is based on increasing their market share until there is basically no-one else left in the market. Once they become the only market (or, the only market that really counts), they are able to extract price concessions from their suppliers. I believe Wal-Mart has followed a similar strategy for decades. (In a wholesale business model, retailers are the market as far as suppliers are concerned.)