Am a bit peeved with a friend who borrowed a book of mine, just after I'd bought and read it (I'd had it about a week). A couple of months later I asked for it back, as my DP's mum wanted to read it, and she said "oh! I haven't finished it yet". Another month or so passes and I ask her again, and this time she says she'd leant it to her own mother, but would get it back.
Another couple of months pass and I was at her flat, and while she was making tea I began looking at her bookshelves. When she came back in the room I asked if she had finished with that book, so I could give it to DP's mum. She said, "is it not on the shelf? I guess flatmate-who-has-just-moved-out must have taken it".
Now I know it is just a £6.99 book, but wouldn't you offer to replace something if you borrowed it and lost it? It seems too petty an issue to ask her to replace it, but it's just annoying that she doesn't think it's the decent and normal thing to do...or maybe it isn't? Maybe IBU.