Greenmantle

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 frien...