A pale horse - Charles Todd

The Great War never relinquished its hold on Scotland Yard Inspector Ian Rutledge, leaving him haunted and isolated, unable to forget. In the spring of 1920, he's dispatched to Berkshire to find a missing man whose war work is so secret even Rutledge cannot know its true nature. Meanwhile, miles away, an unidentified body has been discovered in the ruins of a Yorkshire abbey, clothed in a monk's robe and wearing a gas mask.

In the shadow of a great white horse cut into the chalk hillside-where cottages once built to house the sick and untouchable now shelter outcasts like himself-Rutledge must extract a terrible truth from those who hide from the past. For death is never quite finished with anyone, least of all the men who fought in the bloody trenches of France.