A complete catalogue of the Railway and Airway Letter stamps (including some carried by ships), from their first issue in February 1891 until they had become “philatelic” at the end of 1971. The Railway Letter Service was the only real express letter service, a service wich the Post Office, in spite of its monopoly, could not provide.
The standard reference handbook on these popular items and a priced catalogue which although the prices are out of date does reflect the relative scarcity of each stamp.