i'm someone who takes a lot of care about their books, i especially don't like when the back is wrinkly/broken on paperbacks because of opening them all the way. after all that makes them break down faster in general, over time. and it looks worse in a shelf.
anyways, everyone handles their items differently, which is fine - a friend once borrowed a book from me which was in pristine condition after multiple years of me owning and reading it. when he returned it, the back was completely wrinkled, pages had dog-ears, he had bent the cover all the way around to be able to hold it better etc.
that kind of made me sad because it felt like something i took a lot of care to keep it in good condition was "destroyed" in a matter of weeks. of course i wasn't mad towards my friend because i realized that people handle books differently.
that being said. if someone borrows you something, try to keep it in more or less the same state as when it was given to you. accidents happen, of course. i on the other hand have learned to either not borrow books i care about, or make a point about me taking care of them when reading.
sometimes it's also not as easy as "just buy another copy". some books i care about because i bought them on a vacation, or someone gifted them to me. so the exact item has a sentimental value an additional copy will not be able to replicate. but again, i just learned to create two categories of books in my mind, those i borrow, and those i don't.
after all it's 2026, just download an eBook from Anna's Archive (if they borrow the physical copy from me there's no monetary transaction towards the author as well, so i really don't feel bad about it in these specific situations).