Wikipedia
Moonheart is an urban fantasy novel by Charles de Lint. It takes place in 1980s Ottawa, where Sara Kendell and Jamie Tamson, owners of an antique store, come into possession of a peculiar ring. At the same time Kieran Foy, a wizard of sorts, is searching for his missing mentor, Thomas Hengwr, while simultaneously eluding the newly formed Project Mindreach from the RCMP. When Sara and Kieran run into each other and end up in "the other world," they find that there is much more to Sara and to Tamson House (Sara and Jamie's house the size of a city block) than previously known. But when an ancient evil resurfaces and threatens to destroy both worlds (and all the people within), the officers, wizards, and Tamson House residents have to learn to work together to destroy the monster.