Mary Roberts Rinehart 
Dangerous Days [PDF ebook] 

As America prepares for the Great War, steel magnate Clayton Spencer becomes increasingly aware of the shallowness of his wife and his widening distance from his son, Graham. Graham, who wants to enlist, struggles to overcome his mother's selfish love and his own weak nature. While Clayton starts to fall in love with Audrey, a "real" woman who shares his views on war and enlistment, German workers in his factories seek to sabotage Clayton's munitions plant.
Dangerous Days contained a mixture of mystery and romance. The mystery was due to the espionage and the on-going war effects, and the romance was the heart of the story.