My mum has a very simple solution to this problem. It’s a booklet of temporary bookmarks which say “borrowed from Liz”. The bookmarks have a perforated line which allows them to be ripped out of the booklet, and there’s space in the stub to write the name of the book and the name of the person borrowing it. Every time anyone borrows a book it comes with a bookmark reminding them where they got it from. It works really well.