Amazon says it's now selling substantially more Kindle books than standard hardcover ones, and that its recent price cut has boosted demand for the e-reader dramatically.
The figures put paid to the idea that the iPad will be killing off the Kindle any time soon.
"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," said Jeff Bezos, founder and CEO of Amazon.com.
The price cut left the Kindle costing less than half the price of the cheapest iPad.
"In addition, even while our hardcover sales continue to grow, the Kindle format has now overtaken the hardcover format," said Bezos. "Amazon.com customers now purchase more Kindle books than hardcover books - astonishing when you consider that we've been selling hardcover books for 15 years, and Kindle books for 33 months."
The demand for Kindle books is accelerating. Amazon says that over the last three months as a whole, for every 100 hardcover books it's sold, it has sold 143 Kindle books. Over the last month, though, that has risen to 180 Kindle books for every 100 hardcovers. The figures don't include free Kindle books - and they do even include hardcover books where a Kindle alternative isn't available.
However, sales of hardback books have been on the decline for years; comparative figures for paperbacks - vastly greater sellers - weren't released. E-books tend to come in more cheaply than hardbacks; most of Amazon's cost under $10, less than half the price of a hardback.
The timing of the announcement may be significant. Amazon is due to release its second quarter financial results at the end of the week. It's possible the company wants to make sure that its good news doesn't get overlooked.