Based on true accounts of the life of Joseph Alfred Slade.
I'm not sure how I managed to get a hold of book 3 but not the first 2, so some of it was a bit confusing.
The main character is a lawyer from NYC who moved west and became part of the vigilante group patrolling (near as I could figure) what is now southern Montana down to Northern Nevada during the Civil War. There were no official courts as yet, so they basically policed their own.
Worth the read if you are interested in the old west, but I would suggest starting with book 1. : )