Greenmantle

Full text.
November 1915. Richard Hannay is tasked to undertake a top secret mission to investigate a plot to create a holy war throughout the Muslim world and draw troops and resources from the Western Front. Hanny must journey through war-torn Europe with only his three loyal friends, Peter Pienaar, John S Blenkiron and Sandy Arbuthnot, to help him as he unravels coded messages, escapes murderous mobs and tracks down the mysterious prophet who holds the key to everything - the man known as 'Greenmantle'.
One of the best spy novels of all time, Greenmantle remains to this day a gripping reflection on the power of political Islam.