E. W. Hornung 
No Hero [PDF ebook] 

Wounded warrior Duncan receives a mysterious letter from an old friend requesting his counsel. During a follow-up visit, he presses Elizabeth for details, and she admits that an older woman whose intent could be nefarious has set her sights on Elizabeth’s nineteen-year-old son. Sensing her obvious distress, Duncan agrees to pitch in and help, setting off on an international manhunt with consequences no one could have foreseen.