Valentine Williams 
Okewood of the Secret Service [EPUB ebook] 

No Herr Doktor Clubfoot this time around but the Okewood family returns for another battle with German espionage. The (unnamed) chief of the British Secret Service thinks very highly of Francis Okewood but he’s occupied in spy business on the Continent right now so brother Desmond is tapped to tackle a plot which threatens to unleash squadrons of U-boats onto a convoy transporting American doughboys bound for the Western Front. (Goodreads)