What one reviewer said about Greenmantle (Wordsworth Collection) by John Buchan:
This book had all of the elements of a good spy novel--danger, exotic locations, and neat plot twists. It is about a young mining engineer's espionage adventures during World War I. The book was a real page-turner and I couldn't put it down. However, some of the political information confused me. If you're a World War I buff you might understand some of the technical stuff, though. This book also continues the adventures of one of my favorite characters, Richard Hannay, who first appeared in The Thirty-Nine Steps. He is an extremely likable character and this book is a great one if you like espionage novels and adventure stories. P.S. Read it and you'll see why this would make a great movie.
Download Greenmantle from Project Gutenberg
or find a hard copy