I find this argument overly theoretical and unsupported. Book selling isn't a natural monopoly. If Amazon dumped books and forced out everyone, can they really then use that monopoly status to raise prices? The market has a fairly low barrier to entry and any attempt to have high margin pricing would be met with new competition pretty quickly.

Does that monopoly result in increased cost for the customer? I assume there is harm to the general economy the problem is quantifying that and also convincing people to pay more for something.

But they do so by creating a new monopoly where there publishers set the price.