Written by British author and renowned vintage and classic Harley-Davidson specialist Steve Slocombe, this is the first book to cover the Sport models from 1919-1923, based on the British Douglas motorcycles where the cylinders were positioned in line with the frame. The engine was the first side-valve produced by Harley-Davidson, and the clutch and transmission were integrated in the engine case, giving the bike a cleaner appearance and fewer places for oil to leak. Despite good sales figures in Europe, the great success in the United States failed to materialize due to competition from Indian, and Harley-Davidson discontinued production after 5 years.

This book provides information on the early days of the Douglas Flat Twins, the business environment at the time, detailed technical descriptions of each assembly, and full-page pictures of stunning restored and unrestored machines for each year. The book benefits from numerous in-depth contributions from members of the Antique Motorcycle Club of America (AMCA).