The world is not beautiful
When I loan out my books, I just make it clear that bodily harm will come to the person in proportion to the amount of damage my book suffers. If you return my book in the same condition I lent it to you, you don't need to worry. If on the other hand you break the spine of my book, well, I believe the saying goes: an eye for an eye. D:<
Just kidding... sort of.
But what I'm willing to lend really depends on the person. I buy lots of used books to begin with, and while I try and get the best copy, most of my books come pre-worn. So those I'd probably lend to most of my friends. My rarer and/or nicer condition books though, there's an extremely limited number of people I'd lend them to. My parents are not on that list. Especially my dad. I don't know what he does, because he's not super-abusive to them, but he really messes up books. I loaned him my copy of A Clash of Kings and by the time he was done the cover had fallen completely off, one page was almost torn through completely, and a section of the book started to come apart from the rest. I managed to use my limited book repair skills to patch it up fairly well and it's in readable condition again, but I ended up buying another copy for my personal collection and relegating that one to being my lending copy.
Therefore it is.