I nominate The Thirty-Nine Steps
by John Buchan
Reviews on Amazon say: An effortless adventure classic that spans the void between dime shocker and quality literature. The rapid elaboration of the plot, that is so well known that it has passed many images into popular conciousness, is still satisfying after many reads....It was not until recently that actor Douglas Fairbanks, Jr. and others learned and revealed the information that John Buchan, author of "The Thirty-nine Steps" as well as the highly successful Greenmantle series, had been the head of the secretive British domestic intelligence agency which parallels the FBI in the United States, MI-5. With that knowledge it is increasingly easier to see how the Scotland-born Buchan was able to write such penetrating spy stories, which contain such a strong tone of believability....John Buchan's novel The Thirty-Nine Steps is the prototype of the modern thriller novel
, what he called a "shocker." In it, Buchan introduced Richard Hannay, the prototype of the resourceful, intelligent, and tenacious hero of the modern thriller. And while the story may not be as intricate or exciting as its descendents', The Thirty-Nine Steps still succeeds at what Buchan set out to do--entertain.
It is available for free at MR in the Richard Hannay Omnibus here: