Between Two Thorns by Emma Newman

I love the premise for this novel. Beneath the cities of Bath, London and Oxford there are mirror cities
which exist in the Nether. This is the world between our 'real' world, and the land of Fae. It is a 'Fae-touched society'. Its inhabitants are ruled by the Fae, and once they reach adulthood rarely, if ever, enter our world (which they call Mundanus).
The heroine of the book, Cathy, is a member of one of the most prominent families in Aquae Sulis, which is the mirror city of Bath. She is stifled by her life there, the social rules and etiquette are of the Regency era and it is impossible for a woman to have independence. So she has run away into Mundanus and has spent three years at university in Manchester. However she has been discovered.
At the same time strange events are taking place in Aquae Sulis. One of their most important citizens has gone missing. There are a group of people called the Arbiters whose job it is to police between the Nether and Mundanus. An Arbiter named Max has to find this man, the Master of Ceremonies. Max's sidekick is a stone gargoyle come to life, which I think is a most wonderful thing.
This is a perfectly imagined world. It is pretty and also threatening. I was completely caught up in it. This is the first in a trilogy and I believe the second book has just been published. I can't wait to read it.