Before you dismiss this because it's limited to hardcover books, keep in mind that, for general books, a hardcover version is often released before a paperback one is made available. This means that many new hot books will only be available as hardcover or Kindle editions. And the Kindle format is winning.
This is a remarkable result and should be considered as a sign that we are reaching a tipping point for ebooks, to phrase it in a Gladwell-esque way.
I don't think we're at a tipping point yet. Amazon sold Kindles to its most regular book customers. We'll reach a tipping point when the general public begins to purchase ebooks, but I don't think we're there yet.
We don't know how many of these purchases are to Amazon's best customers. I would be very curious to see the purchases broken down by the number of customers purchasing ebooks vs those purchasing paper books. And by quantity, how many ebooks are purchased vs paper books per customer. Is this simply a fact of frequent customers now buying ebooks, or many more customers now buying ebooks.
This is a remarkable result and should be considered as a sign that we are reaching a tipping point for ebooks, to phrase it in a Gladwell-esque way.