Chain of Gold Novel by Cassandra Clare

Cordelia Carstairs is a Shadowhunter, a warrior trained since childhood to battle demons. When her father is accused of a terrible crime, she and her brother travel to London in hopes of preventing the family’s ruin. Cordelia’s mother wants to marry her off, but Cordelia is determined to be a hero rather than a bride. Soon Cordelia encounters childhood friends James and Lucie Herondale 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.