John Buchan 
Greenmantle [PDF ebook] 

In this 1916 classic by John Bucan, Hannay once again becomes embroiled in a sinister plot that could shake the foundations of the world as he knows it. Whispers of an uprising in the east reach Hannay as he recovers from a hard-fought battle, but rest is the last thing he’ll be allowed. Journeying to Constantinople to meet his friend Sandy, he must once more delve into the sinister machinations of an unseen foe, and thwart a plan that could turn the tide of war against the Allied Powers.