I've since found the book in our local library. The quote isn't exactly as I read at another site, but still speaks of a armed, so-called "Black- Confederate," shooting at Union troops. "On each of our posts was stationed one of Berdan's sharpshooters, who were always on the look out for "game," and woe to the rebel who put himself in their way. One of them who was armed with a telescope rifle had placed a negro picket "Hors de Combat" (disabled ?) the day before. In front of our line stood a large hollow tree, having loop holes cut into it so a rifle could be run through and discharged at our men without danger to the negro who fired it. On this occasion our sharp shooter had fired twice at the black without hitting him, but in the afternoon he left the tree and was taking a walk for the benefit of his legs, when suddenly he was flopped on his face before he had taken six steps. Two white men who with him (a second negro ?) tried to haul him back, but a few doses of leaden pills (the author's favorite term for bullets) being thrown their way, he was left alone until darkness gave them a chance to take him away."

"Gone For A Soldier" page 56-57.