I look after books. It causes me genuine pain to crack a spine or crease a cover. I mean, I read A Suitable Boy without cracking the spine. Whether it's likely to rain or not, I keep a plastic bag in non-waterproof bags to protect books from water damage (I seriously can't bear to read wobbly water damaged books - the awful creaking of the pages!). Of course, I don't think everyone should keep to these freakish standards - unless I lend them a book. I make it very clear if I lend someone a book that I expect the lendee to look after it, and I can think of few ruder things than to treat someone else's book in a way in which they would not treat it themselves.
I don't give a shit if you crack your own books' spines or bend back the cover or write all over them or throw them down the stairs. But if you borrow my books, you look after them. Or you can replace them. And no, I'm not joking. I think if you borrow anything, from a saucepan to a pair of shoes, you treat them not as you treat your own stuff, but as the owner of the borrowed items treats hers. And yes, that goes even if you think the owner is anal or weird. I once loaned someone a book who returned to me a creased, water-rumpled mess without a word of apology. I can't imagine she'd have behaved the same way if I'd loaned her a top and she'd given it back to me in rags.
Is it anal? Yes. Is it verging on OCD? Probably. But they're my books. So I treat them my way. And anyone who borrows them should do the same.