Greenmantle, The World At War

“Greenmantle” is the second of five novels by John Buchan featuring the character of Richard Hannay. It is one of two Hannay novels set during the First World War, the other being Mr Standfast (1919); Hannay's first and best-known adventure, The Thirty-Nine Steps (1915), is set in the period immediately preceding the war. (Excerpt from Wikipedia)