This is why we have a regulated market, and rules about monopolies.
What Amazon would have to do to maintain a monopoly is to continuously locate and destroy startups which try to compete with it, by using its scale. You're saying that, while it's doing this, it may make things worse for the consumer. Perhaps, but that's REALLY tough to do (until someone writes software to do it).
One thing is for sure though - regardless of how good the service is to consumers, the demand for human labor will erode further and the comparative advantage of local economies will erode all over the world - not just for books but tons of products and services. As a result, we will need an ever greater welfare state so that many out-of-work people can be paid to be consumers and send price signals back into the economy, even if they are unable to make much money themselves (because presumably robots would be employed instead).
This problem does not seem to be surmountable with the level of social infrastructure we have. I doubt even more societies with a greater level of public organization, like Norway, could do this. Knowledge and experience are just best when hoarded.
But I imagine that Amazon is continuously under competitive pressure -- consider the disruption e-books caused. Had that caught Amazon flat-footed, it could have decimated their book business.