Mr. Standfast. by John Buchan

Published in 1919, Mr. Standfast is a thriller set in the latter half of the First World War, and the third of John Buchan's books to feature Richard Hannay. Richard Hannay is called back from serving in France to take part in a secret mission : searching for a German agent. Hannay disguises himself as a pacifist and travels through England and Scotland to track down the spy at the center of a web of German agents who are leaking information about the war plans.
He hopes to infiltrate and feed misinformation back to Germany. His journey takes him from Glasgow to Skye, onwards into the Swiss Alps, and on to the Western Front. During the course of his work he's again reunited with Peter Pienaar and John Blenkiron, who both appear in Greenmantle, as well as Sir Walter Bullivant, his Foreign Office contact from The Thirty Nine Steps. The title of the novel comes from a character in John Bunyan's Pilgrim's Progress to which there are many references in the book, not least of all as a codebook which Hannay uses to decipher messages from his allies.
The book finishes with a captivating description of some of the final battles of the First World War between Britain and Germany in Eastern France.