Murder at Manassas — Michael Kilian

A Southern gambler is on the trail of a coldblooded killer as war breaks out between the States in this “enjoyable and entertaining” debut mystery (Historical Novel Society).

It's July 1861, and both the Union and Confederate armies expect to win the war within 24 hours. For Harrison Raines, a southern dandy ensconced in DC society, it's time to choose a side. Though Raines loves Virginia, he hates slavery with a passion. And joining either army would mean leaving Washington and deserting the beautiful Caitlin Howard, a British actress whose heart, sadly, lies with the eccentric thespian John Wilkes Booth. For Caitlin's sake, Raines attempts to remain neutral in a time when neutrality means death.
 
On the morning of the battle, Raines escorts Caitlin to watch the carnage firsthand. When the First Battle of Bull Run turns into a rout, he sees 1 major fighting to rally his troops -- a major who is later found dead far behind Union lines, branded a deserter. To clear the dead man's reputation, Raines must solve a murder as bullets continue to fly.
 
Murder at Manassas is the 1st book in the Harrison Raines Civil War Mysteries, but you may enjoy reading the series in any order.