We shall not sleep - Anne Perry

The fifth, and final, book in Anne Perry's powerful series of novels set during World War One.
The war is coming to a close. Joseph Reavley's regiment has suffered huge losses but all live in hope that peace will soon be upon them, when a further, brutal tragedy strikes the Front. Sarah Price, a young nurse, is savagely murdered and Joseph vows to find the person responsible.
Matthew, Joseph's brother, is already at the Front to meet a prisoner who claims to be able to identify the shadowy Peacemaker, when he becomes implicated in the nurse's death. If Matthew is to bring the prisoner to the office of the Prime Minister, he must work with Joseph to ensure that Sarah Price's killer is found. If Matthew is able to put a stop to the Peacemaker's schemes, there will be a chance for lasting peace; if he fails, then freedom and liberty could be all but a distant memory for future generations...