Amazon revealed today that net sales for the second quarter of 2011 rose 51 percent–reaching $9.91 billion for the quarter.

CEO Jeff Bezos remained coy about Kindle eBook sales, however. According to the release, “sales growth of Kindle devices accelerated” compared to last quarter and the $139 ad-supported Kindle 3G is now the most popular model of the eReader.

Here’s more from the release: “The U.S. Kindle Store now has more than 950,000 books, including New Releases and 110 of 111 New York Times Bestsellers. Over 800,000 of these books are $9.99 or less, including 65 New York Times Bestsellers. Millions of free, out-of-copyright, pre-1923 books are also available to read on Kindle.”