Island of Shadows — Jane Godman

It is 1930 and there is a dark undercurrent to the glamor of the Venetian lagoon and its lavish masquerades. The Ombra are a vampire-like race, living in the shadows. Instead of feasting on blood, they survive by feeding on the souls of the dead. The Ombra exist alongside the living without fear of detection. Having thrived for centuries, they are powerful, wealthy, fear no one and ruthlessly destroy those who stand in their way.
Maggie Forbes has come to Venice to see if she can discover what happened to her husband, Martin, who disappeared on a research trip to the island of Poveglia, the most haunted place on earth. Soon after her arrival in Venice, Maggie makes the acquaintance of two very different men. Torn between friendship and lust, Maggie must follow in Martin's footsteps to unlock the secrets of Poveglia.
But time is running out. The Ombra lord is seeking a bride and the wedding day is looming.