The Mirk and Midnight Hour by Jane Nickerson is set in Mississippi during the American Civil War. This is the second book in the Strands series but it can be read as a stand alone.
From the cover:
Seventeen year old Violet Dancy is left at home in Mississippi while her father fights in the war against the North - a war that has already claimed her twin brother. Grieving and adjusting to life with a new family (his father remarried shortly before his departure) Violet feels a stranger in her own home. She comes across a severely injured Union soldier and although he's an enemy and from the side that killed her brother, she helps him and tends to his wounds. The story is based on the Scottish fairy tale - Tam Lin.
