Amazon.com Inc. said it reached a milestone, selling more e-books than hardbacks over the past three months.

But publishers said it is still too early to gauge for the entire industry whether the growth of e-books is cannibalizing sales of paperback books, a huge and crucial market.

Amazon CEO Jeff Bezos displays a Kindle. Associated Press

In a statement Monday, Amazon's chief executive, Jeff Bezos, also countered the perception that sales of the company's Kindle e-reading device had suffered due to competition from other devices, such as Apple Inc.'s iPad.

He said the growth rate of Kindle device sales had "reached a tipping point," having tripled since the company lowered its price to $189 from $259 last month, following a similar move by competitor Barnes & Noble Inc. to cut the price on its Nook e-reader.

Amazon said Kindle device sales accelerated each month in the second quarter—both on a sequential month-over-month basis and on a year-over-year basis. But the statistics that Amazon shared were all relative—it didn't share actual sales figures. The company has never said how many Kindle devices or e-books it has sold.

Barnes & Noble, the nation's largest bookstore chain retailer, also has "seen a big uptick" since it cut the price of its Nook e-reader, a spokeswoman said.

Sony Corp. , too, said that its sales of e-books were growing steadily, while second-quarter sales of its e-reading devices were about triple the level a year earlier.

Amazon painted a picture of accelerating growth in sales of e-books, which can be read on the Kindle and through software on a host of other devices, including Apple's iPad and iPhone. The figures don't include free e-books.

Over the past month, the Seattle retailer sold 180 Kindle books for every 100 hardcover books it sold, it said.

"That is dramatic evidence of how powerful the e-book is now," said Citigroup analyst Mark Mahaney. "What the iPad and other book reading devices have done is just raise the overall e-book market—and Amazon is extremely well positioned to take advantage of it."

Still, the hardback comparison figure doesn't necessarily mean the end is near for paper books. Amazon said its hardback book unit sales also continued to increase. Moreover, while Amazon has become one of the largest booksellers in the U.S., it still attracts an online audience that is more inclined to be early adopters of new reading technology.

As for the effect on paperbacks, Madeline McIntosh, president, sales, operations and digital at Bertelsmann AG's Random House Inc., said: "Our conclusion is that there's no data to prove any connection—good or bad—between growth in e-books and the growth or decline, in trade paperback sales. ... If anything, we may be seeing a positive effect in which the steady pace of e-book sales helps to keep a book in front-of-mind for a growing number of consumers after hardcover momentum slows."

A second leading publisher said that it is too early to link the increase in Amazon's e-book and hardcover sales to any possible decrease in the sale of paperbacks. "We need a few more months under our belts," said David Young, CEO of Lagardere SCA's Hachette Book Group.

Another statistic shared by Amazon sought to suggest that Amazon remains the leading retailer for e-books. The company said that of the 1.14 million James Patterson e-books sold as of July 6, nearly 868,000 were from Amazon.

In June, Apple CEO Steve Jobs had claimed that his company's iBookstore, which launched in April, had taken 20% of the market.

Write to Geoffrey A. Fowler at geoffrey.fowler@wsj.com and Jeffrey A. Trachtenberg at jeffrey.trachtenberg@wsj.com
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