The Thirty-Nine Steps

The Thirty-Nine Steps is an adventure novel by the British author (and future Governor General of Canada) John Buchan, first published in 1915. It is the first of five novels featuring Richard Hannay, an all-action hero with a stiff upper lip and a miraculous habit of getting himself out of sticky situations.
In May 1914, Europe is close to war and spies are everywhere.

Richard Hannay has just returned to London from Rhodesia in order to begin a new life, when a freelance spy called Franklin P. Scudder calls on him to ask for help. Scudder reveals to Hannay that he has uncovered a German plot to murder the Greek Premier and steal British plans for the outbreak of war. Scudder claims to be following a ring of German spies called the Black Stone.