Richard Hannay, now fighting in the war, makes a second appearance, threading some hazy clues together and chasing across Europe to Constantinople, to defeat a German plot for a Muslim uprising in the Middle East. Femme fatale Hilda von Einem is the criminal mastermind; master of disguise Sandy Arbuthnot plays a key role, and John Blenkiron and Peter Pienaar make key appearances. The wild adventures include battles and a memorable ride with the Cossacks.
Audio version, Librivox: https://archive.org/details/greenmantle_0709_librivox
Available to read at https://www.gutenberg.org/files/559/559-h/559-h.htm
BBC radio adaptation by Patricia Hannah from 2005, https://archive.org/details/john-buchan-greenmantle/1.mp3