We don't charge a patron if the book can still circulate.  It's either usable or it's damaged and we billed the patron for its replacement.  If it's stained, but it doesn't pose a biological (mold, mildew, etc.) hazard, we put a note on the front page noting the type of damage, as well as a date when it was discovered, and the appropriate staff initials.  If it's torn or losing pages, but can be easily repaired, we do that and note the mended date in the same way. If it's not repairable, then we bill for damage.  If it's just circ'ed many, many times, we just call it usage and withdraw it without billing it.