It's only been on the market since 2007, and already, it's outsold a staple on Amazon.com.

The nation's largest online retailer has announced that the Kindle has outsold paperback books as the most popular format on Amazon.com. The news came six months after Amazon said Kindle book sales had overtaken hardcover sales.

Since the beginning of the year, for every 100 paperback books Amazon has sold, the company has sold 115 Kindle books.