The Third Circle. Number 4 in series

Leona Hewitt, disguised in men's formal evening clothes, has secretly made her way into Lord Delbridge's private museum to find a relic stolen from her family. But someone else is in the dimly lit gallery on the same errand: a tall, black-cloaked man whose very voice is enough to put her in a trance. Thaddeus Ware, a mesmerist with psychic gifts, is accustomed to fearful reactions from others - women in particular.
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 crystal they recovered. 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 it, for it is 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 her. With the stolen crystal in their possession, the danger is only beginning...