Aha! There is a caveat next to the list. Apparently the numbers are estimates, especially for the older books. The records on Shakespeare's book sales from the 18th-19th centuries probably aren't too accurate. This may also explain how dear sweet Barbara Cartland wound up on the list. However, most of her books were published as inexpensive paperbacks and goodness knows she was prolific. Romance readers, here's a little validation. There you go.