I don't remember much about it, but I've been looking to find this title again for some time. It's about a young woman who is capable of wielding magic and has a partner in the form of a ferret familiar? Her best friend was trapped in a mirror and she was trying to figure out a way to rescue her. She once had a thing with her friend's brother, but he slayed the main character's previous familiar because he was trying to break the mirror. I might have a few details wrong, but I hope that this is enough to go off of.