Ace High in Wilderness by Rob Hill
It has been seven years since the Civil War ended. Ex-sharpshooter John Wright has tried to forget the horrors of his incarceration in the Confederate POW camp at Andersonville, where he watched, powerless, as one of the guards murdered his childhood friend. Now, two ex-prisoners show up, telling John they have tracked down the murderous prison guard and need help bringing him in. They offer John a share of the reward. But all is not as it seems and John is about to find out that the road to justice is rocky indeed.