Arsène Lupin 8: The Shell Shard (Aka The Woman of Mystery)

The Shell Shard, is the designated eighth book in the Arsène Lupin series by Maurice Leblanc though it does not actually feature Lupin (see  publisher's note within).

As France prepares for the Great War, Paul Delroze recalls the event from his childhood that has scarred him. On a countryside trip with his father, a decorated war veteran from the Franco-Prussian War they came across a group of Germans. Their leader, a cold man with a striking resemblance to the Kaiser, quickly sends the father-son duo on their way. But before they leave, a woman approaches Paul's father and in a sudden thrust of a steel blade kills him in cold blood. Paul escapes but the face of the man is imprinted in his mind. With the two countries at war again Paul is presented with a chance for revenge. The Shell Shard is at once a story of romance, mystery and crime, that will grip present day readers as much it did readers a hundred years ago.