The Thirty-Nine Steps by John Buchan

From chapters.ca:
Richard Hannay, who, despite claiming to be an 'ordinary fellow', is caught up in the dramatic race against a plot to devastate the British war effort. Hannay is hunted across the Scottish moors by police and spy-ring alike, and must outwit his intelligent and pitiless enemy in the corridors of Whitehall and, finally, at the site of the mysterious thirty-nine steps.