"Copperhead" by Bernard Cornwell, Book 2 of 4 of the Starbuck Chronicles

Nate, a Yankee-turned-Confederate, finds his loyalties tested at the Battle of Ball's Bluff 1862

It is the summer of 1862, and the Northern army is threatening to capture Richmond, the Confederate capital. Bloodied but victorious at the battles of Ball's Bluff and Seven Pines, Nate suddenly finds himself accused of being a Yankee spy. Proving his innocence and finding the real spy will require courage and endurance rarely seen even in the brutal fog of war. Failure could mean the fall of Richmond and a career-ending defeat for Robert E. Lee.