Barnes & Noble posted a column on Facebook today:
I Ruined a Borrowed Book, Do I Have to Replace It?
I couldn't even believe this question was being asked. Yes, of course you have to replace it!
One of the things the person says is that the friend he borrowed the book from had already read it -- as if that meant he didn't need to replace the book. Why does that matter? If you borrowed an article of clothing and accidentally ruined it, wouldn't you replace it whether your friend had worn it before or not? Some people like to reread books, and even if they don't, they like to keep them on their shelves. It's not up to you to determine whether the person actually needed to continue owning that book. Your friend entrusted you with their possession, and if you want to keep said friend, you had better prove yourself worthy of their trust!
It's one of the things I like about ebooks, actually -- I can "lend" a book by sharing the file with a friend (B&N DRM allows you to sideload the file onto a friend's Nook or into Bluefire Reader, and just input the credit card number to unlock the book). I don't have to worry about my friend bending, creasing, or otherwise damaging the book. And they can't forget to give it back, either! My days of stressing about the welfare of lent books are gone.
Do you lend books? I know a lot of people just plain don't, or they have rules for who they will lend to and what they expect of their friends. Aside from ruining a book and not wanting to replace it, what else would ensure you'd never lend a book to someone again?