The Bishop's Girl is Rebecca Burns' debut novel and is one of the best historical mysteries I've read this year.
Bishop Anthony Shacklock was killed in France during World War I and buried in the graveyard of a church near the field hospital where he ministered to the injured and dying soldiers. At the end of 1919 when the Bishop’s body is exhumed for re-burial in England, a skeleton wrapped in a canvas bag is found on top of the coffin. The bones are that of a female and DNA tests on a finger bone reveal a familial link to the Bishop. Other than that there are no other clues as to who she was or how she came to be buried in the same grave.