John Buchan 
The Thirty-nine Steps [EPUB ebook] 

Three months of carefree life in London has brought Richard Hannay a terrible boredom. Numerous discussions about the weather, restaurants, horse racing and endless tea parties remained the same throughout the whole period. However, suddenly a miracle happened – on the threshold of his house a mysterious stranger appears, telling incredible stories about German spies (the case takes place before the First World War).
Hannay allows a stranger to live in his house and a few days later finds him dead. Suspicion, naturally, falls on him. Hannay decides to escape…