While the announcement isn't too surprising given the popularity of e-book readers like Amazon's own Kindle, it still comes as somewhat of a milestone. Buying books in data format was basically unknown but a decade ago.

The scope of this accomplishment is made even clearer when one learns that Amazon started selling hardcover and paperback books as far back as 1995. It was only twelve years later, in 2007, that the company introduced its Kindle reader and the e-books that accompanies it. Now, only four years after its inception, Amazon is selling more e-books than hardcover and paperbacks combined.

"Customers are now choosing Kindle books more often than print books. We have high hopes that this would happen eventually, but we never imagined it would happen this quickly," said Jeff Bezos, CEO of Amazon.

The company's Kindle line has also seen a marked rise in popularity since the start of the year thanks to Amazon's new Kindle with Special Offers deal for $114 (R786).

All Kindle books are "Buy Once, Read Everywhere", meaning they'll work on all Kindle devices as well as on a number of other platforms, including iPad, iPhone, PC, BlackBerry, Windows Phone and Android devices. Purchasing e-books also has an obvious positive effect on the environment, as much fewer trees need to be sacrificed in the pursuit of knowledge.