The Mysterious Aviator — Nevil Shute

When Peter Moran, a former World War I pilot, picks up a man on the roadside while driving through a bitter rainy night, he is startled to discover that the bedraggled man is a wartime comrade of his who has just survived a crash landing. As he learns more about his old friend's strange mission, Moran finds himself entangled in treasonous international plots, flying adventures, and tests of both his bravery and his loyalty.