Greenmantle. by John Buchan

Greenmantle is the second of John Buchan's novels to feature Richard Hannay, a Scottish intelligence office in the British army, and as such is the sequel to The Thirty-Nine Steps. The book gives the account of Hannay and his associate's separate journeys through war-torn Europe to Constantinople to thwart an uprising that is poised to throw the Middle East, India, and North Africa into disarray, changing the course of the war.
The book was popular when first published and although it has never been made into a film, the director Alfred Hitchcock was said to prefer Greenmantle to The Thirty-Nine Steps, and considered filming it on several occasions.