If anyone could help me find this book I would really, really appreciate it. It's driving me crazy. I read it a few times, but haven't seen it in forever. The book is probably older than ten years, and I think it was written by a woman. It had a vaguely Victorian setting (not steampunk) and the protagonist was a young man who was a thief (his mentor had died or been murdered) but also a magician of sorts, in as much that he knew how to use some magical objects and understood how they worked. It's frustrating because I really liked it, and I think because of all the set-up I got the impression it might turn into a series. Spoiler'd for length, but also because it contains spoilers about the actual plot.
One of his accomplices was a young woman whose name, I think, started with an M (might have been Madeline), and she was also his lover. The opening of the novel concerns them breaking into an estate during a seance (I think) and they run into a ghoul of some sort underneath the building that it turns out might be responsible for corpses that have been showing up all over the city. They go to visit a friend of theirs who was once a very powerful mage (Ambrose, maybe? I think he was described as having white hair and purple eyes?) but has since become addicted to some sort of drug that has left him really scatterbrained, forgetful, and useless... he has someone who lives with him and looks after him. At some point, this mage falls into a sort of coma nobody can wake him from.
Meanwhile, the main character finds out that someone is trying to kill him and is sending powerful spells/curses after him. He and his female friend go to a cottage out in the middle of nowhere because the person staying there is rumoured to be able to deal with this sort of thing, and that night during a storm they encounter this massive creature that rips the place apart trying to get to them. I think there might have been earthquakes beforehand or tremors or something and they realise that the creature came through the ground after them.
Other distinctive bits:
The main character had a painting in his study that let him look out through an identical painting hanging in the office of a man he believes murdered his mentor. The protagonist is consumed with the thought of revenge or ruining this other person's reputation
I don't really remember if magic was common knowledge or just an uncommon thing... there's a reoccurring character who claims to be able to contact the dead they believe is scamming wealthy old women, and they think he might be behind whatever is after them as well.
At one point, while someone is about to be hanged or something, all the gargoyles surrounding this big building come to life and attack everyone. The female hero is pretending to be a doctor or nurse (I think she is or was an actress, so she's very good with makeup and acting) and she flees into the building with some of the police.
The object that they are after in the opening heist is some sort of orb in a box. The female accomplice fits into the party that is going on at the time and sneaks away to find it, while the main character and his friend are trying to break into the estate from down below through the cellar or something... they come across this big coffin that was just ripped apart or something.
During the latter half of the book, they're trapped beneath the city and hiding in an alcove in this giant cavern while the enormous cannibalistic animated corpse of a fairy (apparently fairies are big and mean) hunts for them.
They find out the reason their mage friend is comatose is because of an invisible ring someone slipped on his toe, because whoever put the cursed object there figured nobody would check his feet.