I'm dubious about this analysis. You seem to be saying that prices were low when Amazon had a monopoly and prices have risen as competition entered the market. That's counterintuitive so perhaps I've missed something.
In other words, Amazon had a niche market that few people competed in. Hence, price ended up being artificially low (same as the online music market in the 90s). Once people realized they could turn it into another distribution channel, competitors exploded and the price of the good adjusted to a more normalized market value.
In economics, a monopoly can go one of two ways. It can either push prices to an extreme high in order to maximize profit, or it can push prices to a low in order to lock competitors out of the market and incentive the purchase of other goods (Kindles in Amazon's case). Once the monopoly broke, the price of the good normalized--which for us means it increased.
Amazon then was forced to raise there prices if they wanted to get the same books that Apple got. So yes as the market has grown and there are more choices the prices have gone up. The publishing industry conspired with each other to get hire prices.