According to Amazon’s Thursday announcement, customers are purchasing e-books on the site in record numbers.

In a statement issued by Amazon CEO Jeff Bezos, e-book sales for Amazon’s Kindle device have skyrocketed since its introduction to the market less than four years ago. They have gained so much popularity that customers are now purchasing more e-books on Amazon than all forms of print books combined. Bezos said that while Amazon had hoped to see customers opt for Kindle books eventually, the company never expected the transition to happen this fast.

The Kindle e-reader has been on the market less than four years. It was introduced in November 2007 and has continued to gain popularity. In July 2010 customers had already chosen e-books for the device over hardcover purchases. A mere six months later they showed their preference for electronic over paperback. This milestone meant that e-book sales were higher than both paperbackl and hardcover combined.

Even as Amazon sees its sale of e-books soar, the electronic versions hold a small percentage of the industry sales. However, it is expected that e-books will have taken over a 20% share of the market within the next couple of years. This is positive news in an industry that has seen some rough times recently.

Sales of e-books as a whole roses 145.7% in the past year. That is compared to an increase of only 6% for hardcover and 1.2% for paperback.

Part of the massive increase in e-book sales can be attributed to the availability of so many readers to choose from. Customers have their choice of the Sony Reader, Barnes & Noble Nook, Amazon Kindle and a variety of tablets including the iPad.  Each device holds its own way to purchase books and some have the ability to borrow from libraries and share with friends.

With Amazon being the only place to purchase books for the Kindle, it makes sense that e-book sales would dominate the site. As the device grows in popularity, Amazon keeps the price low to compete with other devices. According to the statement, Amazon saw three times as many e-book sales this far into 2011 as they had seen at this point in 2010.

Amazon hopes to continue gaining readers by offering a choice in prices for the Kindle. Those who want to purchase can opt for an ad-free device at a cost of $139 or a device with on-screen ads for $114.