Since April 1, Amazon sold 105 books for its Kindle e-reader for every 100 hardcover and paperback books, including books without Kindle versions and excluding free e-books.

“We had high hopes that this would happen eventually, but we never imagined it would happen this quickly,” said Jeff Bezos, Amazon’s chief executive, in a statement. “We’ve been selling print books for 15 years and Kindle books for less than four years.”