Mr Standfast by John Buchan
Recalled from active service on the Western Front, Richard Hannay is sent undercover on a crucial secret mission to find a dangerous German agent at large in Britain. Disguised as a pacifist, Hannay travels from London to Glasgow to the Scottish Highlands and Islands in his search, which eventually ends in a spectacular climax above the battlefields of Europe. John Buchan's inside knowledge of trench warfare and government intelligence lend a formidable realism to this superlative spy story. This is a nail-biting classic from a master storyteller.