The Riddle of the Sands — Erskine Childers; Geoffrey Household

One of the earliest spy thriller novels, The Riddle of the Sands: A Record of Secret Service was written in 1903 by Irish author Robert Erskine Childers. Charles Carruthers, a minor British Foreign Office official, is recruited by an acquaintance to a top secret mission to foil a German invasion plan. The book was somewhat prescient, being published a decade before the outbreak of the First World War.