Phillips Oppenheim, E. (1911) The Double Four, London, (aka. Peter Ruff and the Double Four, Boston, Little, Brown and company, 1912)
The Peter Ruff stories were printed between October 1909 and October 1911 in Pearson’s Magazine (US).
Really a series of detective, adventure stories but they doinclude an encounter with a German spy.
Full text at: http://www.gutenberg.org/files/28091/28091-h/28091-h.htm and https://archive.org/details/peterruffanddou00unkngoog
