If you're in a rich Western country, the time you spend reading a book probably has an opportunity cost much greater than the cost to acquire rights to read the book. Why would you spend 10 hours with a book whose marginal cost is $0, when you could pay $10 and spend those ten hours with a better book?
Wow, never thought of it that way. Although I don't think that'd factor in as much if there's books I want to read on both sides, which I assume is the case. So it's not so much an absolute ranking of books. If it's over a threshold, I want to read it. Given two books I want to read, it might not be that clear that I want to read one over the other, but when you add the cost difference it'd become clearer.
If you're in a rich Western country, the time you spend reading a book probably has an opportunity cost much greater than the cost to acquire rights to read the book. Why would you spend 10 hours with a book whose marginal cost is $0, when you could pay $10 and spend those ten hours with a better book?