Apropos of this discussion, I found the (now rather old - 4 November 2010) article by David Pogue in
Scientific American, quite interesting:
The Trouble with E-Readers
"This past summer Amazon made a shocking announcement: for the first time (and ever since), it sold more electronic books than hardcover ones. Now, that headline should have had half a page of footnotes. Amazon provided only the relative proportions of sales, not the actual quantities. It didn’t mention that its e-books of most best sellers cost a flat $10, compared with, for example, $25 for the same book in hardback. And it didn’t say anything at all about paperback sales (which sell the most of all)."