Book 5 of the Ghosts. Spoilers ahead
Caina is sent on a mission again. Theodosia is being sent with a diplomatic mission, openly as an opera singer, covertly to prevent assassination of either the ambassador or the Lord Governor, and Caina goes with her as her servant.
It starts with her averting one attempt even before they arrive, and continues there. Enlivened by a contact turning into stone, and others following. It involves an old enemy, the secrets of the assassin Kindred, poison, jamming an entrance, someone who is seen sneaking about, a sister, dreams, and more.