The Thirty-nine Steps by John Buchan

Richard Hannay, an ordinary man in London, is drawn into a deadly spy conspiracy after a mysterious stranger is murdered in his apartment. Accused of the crime and on the run, he travels across the Scottish countryside to clear his name and stop a plot that could endanger national security. Along the way, he evades police, outwits spies, and uncovers the secret of the “thirty-nine steps.” The plot moves at a breakneck pace, full of disguises, narrow escapes, and suspense. This is a thrilling espionage adventure that laid the groundwork for modern spy fiction.