Dreaming Josie - Susan Macatee

Civil War reenactor Kyle Dalton has just broken up with his girlfriend.  He's not feeling ready to date ... until he sees the beautiful girl on the battlefield.  She proves elusive ... because she's the shade of Josephine, a woman who lived during the war itself.  Josephine, in turn, thinks he's one of the soldiers she's cared for in the hospital.

This is a very short time-travel romance with an unexpected twist at the end.  Well-executed and entertaining.