The Shiloh Sisters — Michael Kilian

A Union agent hunts a savage double murderer in the midst of a bloody battle in this “exciting” historical mystery (Publishers Weekly).
 
General Grant's army is camped along the Tennessee River, ready to deal a crushing blow to the South. If they are able to capture Corinth, the Confederacy's railways will disintegrate, bringing its army to its knees. But first, Grant must pass through the crucible of Shiloh -- the most fearsome fight of the Civil War.
 
On the eve of battle, a senator's wife appears, begging for permission to cross over to Confederate lines. Then, as Grant's army marches south, he finds the woman's body inside a coffin alongside her twin sister -- a death that offends him just as much as any soldier's.
 
Finding a murderer amid an army of killers takes a subtle touch, and no Union agent is shrewder than Harrison Raines. As the field of Shiloh is soaked with blood, Raines will find the guilty party, as long as the bullets continue to pass him by . . .
 
“With his new sidekick, a French-Canadian Indian named Jack Tantou (a great character whose presence lights up every page he appears on), Raines is drafted into a risky investigation that takes him back and forth across enemy lines. Lots of action occurs before the likable secret agent assembles all the suspects before General Grant . . . a fun history lesson.” -- Publishers Weekly
 
The Shiloh Sisters is the fifth book in the Harrison Raines Civil War Mysteries, but you may enjoy reading the series in any order.