It is in Saint-Denis, outside of Paris, and guidebooks seem to barely mention it, if at all. It is an easy Metro ride from the center of Paris, and very worth the trip. Besides being a gorgeous cathedral, it is the burial place of French royalty, including Marie Antoinette. I practically had the place to myself, which was amazing. It is beautiful inside and out, with beautiful carvings and monuments, lovely stained glass, and some nice gargoyles. A definite must-see, particularly if you are interested in French history.