Amazon announced that the firm is now selling more Kindle electronic books than printed books, surpassing hardcover and paperback books combined.
"We had high hopes that this would happen eventually, but we never imagined it would happen this quickly," said Amazon boss Jeff Bezos.
The company revealed that since the 1st of April, Kindle books had sold 105 to every 100 print books. The figure includes books sold that are unavailable in Kindle versions.
The popularity of the firm's third-generation Kindle e-reader doubtless played a role in e-books selling three times as many in 2011 as they did in the same period in 2010.
Amazon recently launched an even less expensive advertising-supported Kindle in the US called the "Kindle with Special Offers". Aggressively priced at $114, Bezos said the cheaper entry had "quickly become the bestselling member of the Kindle family."