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, the Wall Street Journal reports.

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

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