Nobody's Man by E. Phillips Oppenheim (1866 - 1946)

Brigadier general Andrew Tallente, late of Parliament, is implicated in the death of his male secretary, the son of a classmate at Eton. Seems the younger man may have not only been having an affair with the hero's American wife, but had stolen incriminating political documents. A political coup of sorts develops as the opposing party invites Tallente to lead them. Enter the lovely heiress-next-door, who becomes the hero's champion, and perhaps, new flame. (Summary by Matt Pierard)