With respect to fossil fuels, water use and mineral consumption, the impact of one e-reader payback equals roughly 40 to 50 books. When it comes to global warming, though, it’s 100 books; with human health consequences, it’s somewhere in between.
Two New York Times links in a row. The above is a spoiler, by the way, but the journey is more interesting than the destination. I actually expected the number of books required to "break even" would be much higher, I am pretty close to 100 books read on my Kindle already.
Also, they totally wrote this thing about the Kindle, and then changed the title to iPad, right?