The Third Circle / Amanda Quick.

Enhanced descriptions from Syndetics:

New York Times–bestselling author Amanda Quick’s new Arcane Society novel reveals the passionate— and paranormal—secrets of proper Victorian London. Called “supremely addictive” by Booklist, the Arcane Society novels, written under both the Amanda Quick and Jayne Ann Krentz names, have pulled readers into a saga filled with steamy romance, drama, and dark intrigue. Her newest novel promises to mesmerize as well. Leona Hewitt, disguised in men’s formal evening clothes, has secretly made her way into Lord Delbridge’s private museum to retrieve a relic stolen from her family. But someone else is in the dimly lit gallery on the same errand: a tall, blackcloaked man whose very voice is enough to cause her to fall into a trance. Thaddeus Ware, a mesmerist with psychic gifts, is accustomed to fearful reactions from others—women, in particular. After all, a man who can control the minds of others could rob a lady of her virtue—completely unbeknownst to her. But Leona shows no trace of hysteria in his presence. A gifted crystal worker, she exerts a rather hypnotic power over the hypnotist himself. And she is determined to keep the coveted crystal they manage to recover by giving him the slip at a run-down London inn. Thaddeus, on assignment for the Arcane Society, knows the menace Leona is courting by absconding with the crystal. A source of remarkable energy, it holds the potential for great destruction. Lord Delbridge has already killed to acquire the crystal, his key to membership in the elite, shadowy group known as the Third Circle. And, with the help of a ruthless hunter of preternatural skill—dubbed the Midnight Monster by the press—Delbridge intends to find Leona. With the stolen crystal in their possession, the danger is only beginning.