John Buchan 
Greenmantle 

In John Buchans second book focusing on the exploits of the ever resourceful Richard Hannay, we find the English secret agent deep behind enemy lines as World War One rages. This time Hannay is tasked by a senior intelligence officer with investigating a possible plan by the Germans to cause a revolt in the Muslim world that threatens Great Britains war effort. Aided by some friends, Hannay travels across war-torn Europe to Turkey as he attempts to decipher a cryptic clue that may hold the key to the plot and disrupt the dastardly plan before it is too late. One of Buchans most popular books Greenmantle is also one of only two Hannay novels set during the First World War.