Hey, I'm a librarian. If the library has a copy of its collection development policy online (as some libraries do) it might be worth a read. They pretty much always outline the library's policy on donations. Most libraries won't add donations to their collection because space is an issue, but if you donated an item that there is an existing catalog record for (which is very cool of you by the way) which indicates the item is merely missing, I imagine that could change things.