The Last Hours: Chain of Gold, Hardcover, by Cassandra Clare

Cordelier Car stairs is a Shadow hunter, a warrior trained since childhood to battle demons. When her father is accused of a terrible crime, she and her brother travel to Edwardian London in the hope of preventing the family's ruin. Cornelia's mother wants to marry her off, but Cordelier is determined to be a hero rather than a bride. Soon Cordelier encounters childhood friends James and Lucia Heron dale and is drawn into their world of glittering ballrooms, secret assignations and supernatural salons, where vampires and warlocks mingle with mermaids and magicians. All the while, she must hide her secret love for James, who is sworn to marry someone else.