High Hearts, by Rita Mae Brown
Geneva Chatfield, young and boyishly slim, has no sooner married Nash Hart than the War Between the States erupts and he rides off to war. After a short time of staying home alone, she decides to join him. Dressing as a man, she rides off and in short order finds herself again at his side. But there’s a small problem: his commanding officer takes a liking to the new young "man", and Nash has, even before she arrives, taken an active dislike to his commanding officer. Geneva finds herself having less respect for her husband than she does for Captain Mars Vickers.

Paperback, 429 pages