Circle of Dishonor — Sarah E. Glenn

Obsessed with finding the Knights of the Golden Circle, a secret society responsible for her brother's death, former Pinkerton operative Nessa Donnelly assumed his identity and followed his killers to Lexington. The trail ended there, and fourteen years have gone by.  
When local prostitute Belle Brezing turns to her for help, Nessa is immersed in a murder case that threatens to expose all of her secrets. She is dealing with more than murder. The KGC is active again in Kentucky, and bodies are piling up. If Nessa doesn't find the killer fast, she stands to lose everything-maybe even her life. 
 
[Gwen] Mayo's depiction of late-19th century Kentucky is fascinating, and the mystery is ably handled. The author has a knack for the telling detail, giving us a picture of a society struggling to recover from the consequences of the Civil War. Recommended.