Daughter of the regiment : a novel / Stephanie Grace Whitson.

Enhanced descriptions from Syndetics:

Irish immigrant Maggie Malone wants no part of the war. She'd rather let "the Americans" settle their differences-until her brothers join Missouri's Union Irish Brigade, and one of their names appears on a list of injured soldiers. Desperate for news, Maggie heads for Boonville, where the Federal army is camped. There she captures the attention of Sergeant John Coulter. When circumstances force Maggie to remain with the brigade, she discovers how capable she is of helping the men she comes to think of as "her boys." And while she doesn't see herself as someone a man would court, John Coulter is determined to convince her otherwise.

As the mistress of her brother's Missouri plantation, Elizabeth Blair has learned to play her part as the perfect hostess-and not to question her brother Walker's business affairs. When Walker helps organize the Wildwood Guard for the Confederacy, and offers his plantation as the Center of Operations, Libbie must gracefully manage a house with officers in residence and soldiers camped on the lawn. As the war draws ever closer to her doorstep, she must also find a way to protect the people who depend on her.

Despite being neighbors, Maggie and Libbie have led such different lives that they barely know one another-until war brings them together, and each woman discovers that both friendship and love can come from the unlikeliest of places.