John Buchan 
Mr. Standfast [EPUB ebook] 

During the later years of the First World War Brigadier-General Hannay is recalled from active service on the Western Front to undertake a secret mission hunting for a dangerous German agent at large in Britain. Hannay is required to work undercover disguised as a pacifist, roaming the country incognito to investigate a German spy and his agents, and then heads to the Swiss Alps to save Europe from being overwhelmed by the German army.