Amazon.com Inc said that after less than four years of selling electronic books, it's now selling more of them than printed books.
The online retailer said that since April 1, it has sold 105 e-books for every 100 printed books, including printed books for which there is no electronic edition.
The comparison excludes free e-books, which would tip the scales further if they were included.
Printed books include both hardcover and paperback books. Amazon said in July that e-book sales had outstripped hardcover sales. It's now selling three times as many e-books as it did a year ago.
Analysts estimate that Amazon accounts for about two-thirds of US e-book sales.
Amazon also said the latest and cheapest version of its Kindle e-book reading device is the best-seller of the line, five weeks after it was introduced.
The Kindle with Special Offers costs US$114, and shows ads in standby mode.