Amazon sold three times as many Kindle books in the first half of 2010 as it did in the first half of 2009.  It was revealed earlier this week that Amazon is now selling more Kindle books than hardcover books. This was reiterated in the company’s earnings announcement today.

Over the past three months, for every 100 hardcover books Amazon.com has sold, the company has sold 143 Kindle books, Amazon says. In addition, over the past month, for every 100 hardcover books Amazon.com has sold, the company has sold 180 Kindle books.

This is across Amazon.com’s entire U.S. book business and even includes sales of hardcover books where there is no Kindle edition. It does not include free Kindle books.

"The Association of American Publishers’ latest data reports that e-book sales grew 163 percent in the month of May and 207 percent year-to-date through May," Amazon says. "Kindle book sales in May and year-to-date through May exceeded those growth rates."

Five different authors have sold over 500,000 Kindle books. These include Charlaine Harris, Stieg Larsson, Stephenie Meyer, James Patterson, and Nora Roberts. James Patterson has sold 1.14 million e-books to date, 867,881 of which were Kindle books.

During this quarter, Amazon launched Kindle for Android, which should further fuel the service as Android itself continues to grow rapidly in market share. Kindle does face increasing competition, however. Barnes & Noble’s Nook, which just launched an Android version today recently got into something of a price war with Amazon’s Kindle.

According to Amazon, the U.S. Kindle Store has over 630,000 books, including new releases and 106 of 110 New York Times Best Sellers. Over 510,000 of them are $9.99 or less.