Less than four years after their introduction, Kindle electronic books are now outselling physical books on Amazon.

Since April 1, for every 100 print books Amazon has sold — including paper books for which there are no Kindle books available — it has sold 105 Kindle books. The company has sold more than three times as many Kindle books in 2011 as it did in the same time period in 2010.

News of the coup comes less than a year after Amazon announced that sales of its digital book format had surpassed hardcover sales, and less than four months after Kindle sales surpassed paperback sales.

"We had high hopes that this would happen eventually, but we never imagined it would happen this quickly,” said Jeff Bezos, Founder and CEO in a statement.