The London Transport Museum has a short Harry Stevens bio:
With no formal art training, Harry Stevens started his design career in the exhibitions and display trade. He went on to become a prolific freelance commercial artist, specialising in poster design. He worked for a wide variety of clients and agencies and in 1963 won the Council of Industrial Design Poster Award. In 1955 he joined the Society of Industrial Artists and was elected Fellow ten years later. He was also a member of the Society of Modern Painters.Beyond that it's hard to find any addition info, but Harry Stevens' art work speaks for itself.