John Buchan 
Mr Standfast 
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.