Amazon.com, the largest online retailer in the United States, says it has been selling more ebooks than printed books since April 1. 

For every 100 printed books - hardcovers and paperbacks combined - which are sold, it is selling 105 Kindle e-books. This figure excludes free e-books. 

Customers are now choosing Kindle books more often than print books, said Amazon CEO Jeff Bezos.

We had high hopes that this would happen eventually, but we never imagined it would happen this quickly. 

Kindle sales have contributed to Amazon's most rapid year-over-year growth for Amazon's U.S. book business, in units and dollars, in more than a last 10 years.

Also, Kindle book sales have increased three-fold so far this year. As of March, 2011, the stated library numbers over 850,000 titles for ebook on Amazon. (Kindle was launched in November 2007) 

According to Caris & Co. analyst Sandeep Aggarawl, Amazon could see $5.42 billion in Kindle-generated revenue in 2011, a figure that could surge to at least $7.96 billion in 2012. 

Amazon does not make public the number of Kindles sold or e-books sold.