NEW YORK (CNNMoney.com) -- In the latest sign that readers are flocking to e-books, Amazon.com said Monday that it is now selling more Kindle books than hardcovers.
Over the past three months, for every 100 hardcover books Amazon.com has sold, it sold 143 Kindle books. That trend accelerated even more over the last month, when Kindle books outsold hardcovers by a 180-to-100 margin.
That number comes with an important caveat: Amazon didn't disclose how Kindle e-book sales compare to its entire bookstore sales. With most Kindle books priced at $9.99, those e-books are often less expensive than the same title in hardcover, especially for popular new releases. But less-expensive paperbacks make up the vast majority of Amazon's book sales.
On Christmas Day last year -- when e-book readers were a popular gift -- customers for the first time purchased more Kindle books than physical books, Amazon announced in late 2009.
Amazon (AMZN, Fortune 500) also on Monday touted a boom in sales of its Kindle device. The company slashed the cost of the Kindle from $259 to $189 last month, and "reached a tipping point" with that price tag, Amazon founder and CEO Jeff Bezos said. Amazon's Kindle device sales growth rate has tripled since the price cut, he said.
But Amazon has never disclosed how many Kindles it has actually sold, and again declined to do so on Monday.
Amazon is scheduled to report its second quarter sales and earnings on Thursday. 
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