Turncoat by Aaron J. Elkins

It begins with the appearance of a stranger at his door -- an unwelcome visitor ranting madly about money, death, and forgiveness; a man known all too well to Pete's distraught wife, Lily: her father. The next day the man is dead -- and Lily disappears, leaving a note behind begging Pete not to follow. Now, with a business card from an antiques dealer in Barcelona as his only lead, Pete Simon embarks on a twisted and perilous journey that will carry him to places where the hideous crimes of the Nazi aggressors remain fresh in the minds of those who cannot forget ... or forgive. But each door he opens leads him deeper into a painful and shocking past. And suddenly an ordinary American has become more than a concerned husband and seeker of a bitter truth; he has become the target of desperate, dangerous men and their terrifying vengeance.

A haunting parable of good and evil and all the shifting shades of humanity in between, by the Edgar Award-winning master of suspense.