Amazon.com Inc. said this week that it is selling more digital books for its Kindle e-reader than printed volumes.

Since April 1, Amazon, No. 1 in the Internet Retailer Top 500 Guide, has been selling 105 Kindle books for every 100 printed books sold. The conventional book figure includes sales of hardcovers and paperbacks that have no Kindle edition. The e-book figures exclude free Kindle books. Amazon introduced the Kindle in late 2007.

“Customers are now choosing Kindle books more often than print books. We had high hopes that this would happen eventually, but we never imagined it would happen this quickly,” says Amazon CEO Jeff Bezos.

So far this year, Amazon has sold three times as many Kindle books as were sold during the same period in 2010. Amazon did not release specific sales figures.

The retailer went on to say that sales of Kindle with Special Offers, introduced about five weeks ago, has become the bestselling Kindle device.  Kindle with Special Offers costs $114, which is $25 less than the base Kindle e-reader. The cheaper devices display advertising and have sponsored screensavers.