In Amazon’s ongoing effort to show how popular the Kindle and e-books are without giving away real numbers the company has released a batch of new statistics about the two products ahead of its second quarter earnings report set for Thursday. According to the company, sales of the e-reader “accelerated each month in the second quarter—both on a sequential month-over-month basis and on a year-over-year basis.”

“We’ve reached a tipping point with the new price of Kindle—the growth rate of Kindle device unit sales has tripled since we lowered the price from $259 to $189,” declared Jeff Bezos, Amazon CEO. “In addition, even while our hardcover sales continue to grow, the Kindle format has now overtaken the hardcover format. customers now purchase more Kindle books than hardcover books.”

According to Amazon, over the past three months, for every 100 hardcover books it has sold, the e-tailer has sold 143 Kindle books. Over the past month, for every 100 hardcover books Amazon has sold, it has sold 180 Kindle books. The ratio is across Amazon’s entire U.S. book business and includes sales of hardcover books where there is no Kindle edition. Free Kindle books are excluded from the totals.

Amazon e-book sales tripled in the first half of 2010 compared to 2009 and its growth rate in the first five months of 2010 was higher than the 207% increase reported by the AAP. Five authors—Charlaine Harris, Stieg Larsson, Stephenie Meyer, James Patterson, and Nora Roberts—have each sold more than 500,000 Kindle books and Amazon said that of the 1.14 million e-books that Hachette said James Patterson has sold, 867,881 were for the Kindle.

The Kindle store now has more than 630,000 books and over 510,000 are $9.99 or less.