In Secret — Robert W. Chambers / Robert William Chambers

A wartime romance assembled from a jumble of styles. Miss Evelyn Erith lives in a mansion, has a chauffeur and a butler, has a talent for cryptography and works for the government reading other people's mail. Her boss is becoming interested in her.
When the hero arrives the story takes a serious turn into fighting alcoholism. Then a stint at his estate in Scotland, followed by a long grim period fighting the war as secret agents in the forests of neutral Switzerland.