The company says its Kindle book sales have tripled so far this year, and its combined Kindle and printed books business in the United States is now growing faster than it has in 10 years, USA Today reported Friday.
Kindle book sales surpassed hardcover book sales in July 2010, and have now overtaken paperback books to become the most popular format on Amazon.com, the company said in a release.
"Customers are now choosing Kindle books more often than print books," Amazon founder and CEO Jeff Bezos said.
"We had high hopes that this would happen eventually, but we never imagined it would happen this quickly -- we've been selling print books for 15 years and Kindle books for less than four years."
The U.S. Kindle Store now has more than 950,000 books, the company said.
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