Buchan's literary reputation rests chiefly on his fast-paced thrillers, which combine suspense, vivid landscapes, and moral seriousness. His masterpiece, The Thirty-Nine Steps (1915), introduced the resourceful hero Richard Hannay, who reappeared in sequels including Greenmantle (1916), Mr Standfast (1919), and The Three Hostages (1924). Other popular novels include Prester John (1910), an adventure set in South Africa, and The Power-House (1916). The historical novel Witch Wood (1927) revealed his deeper interest in Scottish history and faith.
Beyond fiction, Buchan was a prolific historian and biographer, producing works such as Montrose (1928) and biographies of Sir Walter Scott and Julius Caesar. His final novel, Sick Heart River (1941), exploring redemption in the Canadian wilderness, is regarded by many critics as his finest achievement.