I'm working my way through the entire Benjamin January series; this is the third one.
In this book, January's sister, Olympe, stands accused of murder. A free woman of color's husband has disappeared, and the whispers are that Olympe was paid to poison him. Because she's a voodooienne, the populace is willing to believe it despite any evidence to the contrary.
January and his unlikely compatriot, American police lieutenant Abishag Shaw, decide that they are going to look deeper into the matter. In the meanwhile, Olympe waits in the jail at the Cabildo ... and more people are dying or disappearing. When Marie Laveau makes it clear that she can assist in the investigation, January knows there is more to it than meets the eye.
This series of historical mysteries is quite well-researched, giving the readers a look at Jacksonian-era New Orleans that no other fiction writer I've found has provided. The class distinctions are dealt with frankly, and even the fashion notes are spot-on (like Benjamin January, I think the fashion in ladies' sleeves at the time is horrid).
Highly recommended for intelligent, well-developed mysteries and more than a smattering of historical information.