Does it? For a business with a low barrier to entry like selling books? I'd like to see some examples.

The reason Amazon can have a monopsony is because it keeps prices low. It retains customers and forces publishers to go through them. If they raised prices, they'd lose their advantage, and there are plenty of competitors ready to take the market, including well funded companies (e.g. Apple and Google).

If this was what's usually called a "natural monopoly", maybe that could be argued, but book selling - and especially ebook selling - is anything but.