According to Wikipedia, "Greenmantle is the second of five novels by John Buchan featuring the character of Richard Hannay, first published in 1916 by Hodder& Stoughton, London. Hannay is called in to investigate rumours of an uprising in the Muslim world, and undertakes a perilous journey through enemy territory to meet up with his friend Sandy in Constantinople. Once there, he and his friends must thwart the Germans' plans to use religion to help them win the war, climaxing at the battle of Erzurum. It (Greenmantle) 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. John Buchan, 1st Baron Tweedsmuir (26 August 1875 11 February 1940) was a Scottish novelist and Unionist politician who served as Governor General of Canada. Buchan's 100 works include nearly thirty novels, seven collections of short stories and biographies of Sir Walter Scott, Caesar Augustus, and Oliver Cromwell. Buchan's most famous of his books were the spy thrillers (including) The 39 Steps (which was converted to a play as well as an Alfred Hitchcock movie starring Robert Donat as Richard Hannay, though with Buchan's story much altered.) The "last Buchan" (as Graham Greene entitled his appreciative review) was the 1941 novel Sick Heart River (American title: Mountain Meadow), in which a dying protagonist confronts in the Canadian wilderness the questions of the meaning of life. The insightful quotation "It's a great life, if you don't weaken" is famously attributed to Buchan, as is "No great cause is ever lost or won, The battle must always be renewed, And the creed must always be restated."