The marginal cost is a lot less for an ebook, though. Sure, the sunk costs are the same (actually writing the book), but the costs to produce, distribute and store the book are significantly less, so I'd expect a discount of a few dollars. And yes, also because I'm foregoing the ability to lend it out or resell it. Oh, also the technology is still pretty new, so I still don't think you're getting the same experience as with a copy of one of Knuth's books, or another well-made hardback.