The Ghosts, book 3. Spoilers ahead for earlier ones.
Caina is investigating slavers, and her work frees some and more importantly turns up documents. It also reveals that the slavers had sorcerous help.
So, investigation begins. It involves Ark acting as a guard again, a jewel merchant and his daughter, a man who thinks his legitimate brother's wife did something to her husband, women being told that other people are holding them back, rich widows, a legend of a child-killing spirit, and more.