A Killing at Balls Bluff — Michael Kilian

“A tightly constructed, well-written, and suspenseful whodunit” starring “a relentless but all-too-human hero” (Booklist).
 
The messenger finds Harrison Raines in one of the finest gambling halls in Washington. As usual, Raines is losing. Union intelligence demands his presence immediately -- it's a matter that could affect the outcome of the Civil War -- but Raines delays. After all, he's holding four eights, and as a southern dandy who renounced his family to serve the Union government as a secret agent, Raines can't resist a bet.
 
But as soon as he finishes this hand of poker, Raines will be gambling with more than cards -- he'll be wagering his life. Abraham Lincoln is a close friend of Colonel Baker, and he orders Raines to guard the colonel on the battlefield. But in the chaos of Ball's Bluff, Baker refuses to take cover from enemy fire. When Baker cut down by a Confederate riding a white horse, Raines is a prime suspect for the murder, and must clear his name or risk being a fugitive from both sides of the Civil War.
 
“Kilian's use of historical detail is accurate and pertinent without detracting from what is, essentially, a tightly constructed, well-written, and suspenseful whodunit. Raines, a relentless but all-too-human hero, is an intriguing character . . . in what promises to be a fine series of novels. Both Civil War and mystery fans will appreciate Kilian's grasp of the genres of historical fiction and mystery.” -- Booklist
 
A Killing at Ball's Bluff is the second book in the Harrison Raines Civil War Mysteries, but you may enjoy reading the series in any order.