The Thirty-Nine Steps by John Buchan

Adventurer Richard Hannay, just returned from South Africa, is thoroughly bored with London life—until he is accosted by a mysterious American, who warns him of an assassination plot that could completely destabalise the fragile political balance of Europe. Initially sceptical, Hannay nonetheless harbours the man—but one day returns home to find him murdered... An obvious suspect, Hannay flees to his native Scotland, pursued by both the police and a cunning, ruthless enemy. His life and the security of Britain are in grave peril, and everything rests on the solution to a baffling enigma: what are the 'thirty nine steps?'