Two Storm Wood: Philip Gray

The guns are silent. The dead are not.
1919. On the battlefields of northern France, the guns of the Great War are silent. Special battalions now face the task of gathering up the dead for mass burial.
Amy Vanneck's fiance is one soldier lost amongst many. She heads to France, determined to discover what became of the man she loved.
Meanwhile, Captain Mackenzie cannot bring himself to go home until his fallen comrades are laid to rest. His task is upended when a gruesome discovery is made beneath the ruins of a German strongpoint.
It soon becomes clear that what Mackenzie has uncovered is a war crime of inhuman savagery. As the dark truth leaches out, both he and Amy are drawn into the hunt for a psychopath, one for whom the atrocity at Two Storm Wood is not an end, but a beginning.