John Buchan 
Mr Standfast [EPUB ebook] 
The 2nd Sequel to The 39 Steps

Mr Standfast is the third of five Richard Hannay novels by John Buchan. Set in the later years of World War I, 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. He is forced to work undercover disguised as a pacifist, roaming the country incognito to investigate the deadly spy and his agents.