Amazon.com explained . . .

. . . and a fairly decent overview of the book business at the New Yorker here.

Excerpt:

 Publishers’ dependence on Amazon, however unwilling, keeps growing. Amazon constitutes a third of one major house’s retail sales on a given week, with the growth chart pointing toward fifty per cent. By contrast, independents represent under ten per cent, and one New York editor said that only a third of the three thousand brick-and-mortar bookstores still in existence would remain financially healthy if publishers didn’t waive certain terms of payment.