I shall be near to you : a novel / Erin Lindsay McCabe.

Enhanced descriptions from Syndetics:

An extraordinary novel about a strong-willed woman who disguises herself as a man in order to fight beside her husband, inspired by the letters of a remarkable female soldier who fought in the Civil War.
   Rosetta doesn't want her new husband Jeremiah to enlist, but he joins up, hoping to make enough money that they'll be able to afford their own farm someday. Though she's always worked by her father's side as the son he never had, now that Rosetta is a wife she's told her place is inside with the other women. But Rosetta decides her true place is with Jeremiah, no matter what that means, and to be with him she cuts off her hair, hems an old pair of his pants, and signs up as a Union soldier.
   With the army desperate for recruits, Rosetta has no trouble volunteering, although she faces an incredulous husband. She drills with the men, proves she can be as good a soldier as anyone, and deals with the tension as her husband comes to grips with having a fighting wife. Rosetta's strong will clashes with Jeremiah's while their marriage is tested by broken conventions, constant danger, and war, and she fears discovery of her secret even as they fight for their future, and for their lives. Inspired by more than 250 documented accounts of the women who fought in the Civil War while disguised as men, I Shall Be Near To You is the intimate story, in Rosetta's powerful and gorgeous voice, of the drama of marriage, one woman's amazing exploits, and the tender love story that can unfold when two partners face life's challenges side by side.