Well, 'The Good Lord Bird' by James McBride can be considered a great civil war horror novel. It shows the brutality and the inhumane aspects of the Civil War in a very real and somewhat horrifying way. Then there's 'Cold Mountain' by Charles Frazier. The hardships and terrors that the characters endure during the war give it a horror - like quality. Also, 'The Night Marchers' has elements that blend the Civil War setting with horror themes.
I think 'March' is a notable civil war horror novel. It gives a detailed account of the war and the horror that came with it, like the violence and the upheaval. 'Wicked River' is another. It shows the perils of the Civil War period, with elements that can be truly horrifying such as the danger lurking in the rivers and the wilderness. 'The Widow of the South' also stands out. It tells a story full of horror within the context of the Civil War, with the destruction and the loss that the characters experience.