What's the Name of That Book??? discussion

This topic is about
A Test of Wills
SOLVED: Adult Fiction
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SOLVED. Shortly after WWI, a deserter feels haunted by the officer who didn't kill him [s]
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All she recalls is the plot.
The book takes place shortly after World War I. In back story, this man was a soldier who was suppose to have been shot for deserting. He was not shot and the officer (?) who let him go made an impression on him.
Now, after the war, and as a police officer, he feels the man who did not shoot him is always with him in spirit form and helping him out, like a shoulder guardian angel. He will even help him with solviing cases (possibly, not really sure if that is part of the plot).
And that is all in terms of description she can recall and that I have to go off of. But if it sounds familiar and you think you know what book she may mean, please suggest it and I will tell her and see if we can't track this book down.
Thanks