Only yesterday I noticed a small (5 title) non-fiction publisher doing just that: hardback: $20; e-book: $10, both: $25

I've long said that if Terry Pratchett were available in e-book I'd buy the whole lot - because when you talk about books it's so much easier to search a file so you can find the references; and while I'm not, and won't be for a while, read e-books all the time (the 'in the bath' problem) I'd love to have the book on the go available both as paperback and electronically. Leave the house, continue to read the same book sounds great.