For every 100 copies of a physical book Amazon sell, they sell 48 for the Kindle, where available

"For every 100 copies of a physical book we sell, where we have the Kindle edition, we will sell 48 copies of the Kindle edition. It won’t be too long before we’re selling more electronic books than we are physical books. It’s astonishing."
Source: Jeff Bezos of Amazon, Interviewed by the New York Times, 2nd December 2009
Another stat from the same source:
"When we launched Kindle two years ago, it was 90,000 titles, and today it’s more than 350,000. We’re adding thousands of titles every week. Our vision is every book ever printed in every language, all available within 60 seconds."