The Thirty-Nine Steps by John Buchan

The Thirty-Nine Steps is an adventure novel by the Scottish author John Buchan. The Thirty-Nine Steps first appeared as a serial in Blackwood's Magazine in August and September 1915 before being published in book form in October that year by William Blackwood and Sons, Edinburgh. The Thirty-Nine Steps is the first of five novels featuring Richard Hannay, an all-action hero with a stiff upper lip and a miraculous knack for getting himself out of sticky situations.
The novel The Thirty-Nine Steps formed the basis for a number of film adaptations.
The novel is set during May and June 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.
A few days later, Hannay returns to his flat to find Scudder murdered. If Hannay goes to the police, he will be arrested for Scudder’s murder. Hannay decides to continue Scudder’s work and his adventure begins. He escapes from the German spies watching the house and makes his way to Scotland, pursued both by the spies and by the police.
The mysterious phrase Thirty-Nine Steps first mentioned by Scudder becomes the title of the novel and the solution to its meaning is a thread that runs through the whole story. (Summary by Wikipedia)