In actuality, it is the publishers who have a monopoly on each book that they sell. If Amazon (or any other retailer) wants to sell Amy Poehler's new bestseller Yes Please, they can only buy it from HarperCollins. They can set any wholesale price they want, and no other publisher is allowed to offer a competing price for that book.
That's almost like saying Amy Poehler has a monopoly on her written words. If HarperCollins (or any other publisher) wants to publish her words in a book, they can only pay her for that privilege. She can agree to any terms she wants, and no other person is allowed to agree to a competing offer for those words.
But the point is that while publishers get to sell books through multiple retailers, Amy Poehler does not (in practice) get to license her words to multiple publishers.
