The Adventures of Mr. Joseph P. Cray by E. Phillips Oppenheim

Joseph P. Cray, an American manufacturer who has just completed a year serving coffee to the troops in France during World War 1, returns to London, dons civilian garb, and is soon joined by his daughter, the beautiful Lady Sara Sittingbourne, who lives in London. Together the two seek adventure in the form of crimes foiled, jewels recovered, spies uncovered, and plots smashed.