At Bull Run — Paul Westwood

The second book in the Blackwood Series. A wealthy man hires Jack Blackwood to find the murderer of his only son, who had recently joined a newly-formed Union cavalry regiment. In a city crowded with temptation, the investigation uncovers a killer who is targeting prostitutes and soldiers alike, causing panic in the ranks. Only Jack's wits and the power of the Colt can put a stop to the killings.