 Three titles – "The Forbidden Tower", "The Mystery of the Swaying Curtains", and "The Shadow on the Door" – were dropped from the series for reasons which are unclear.The series was re-issued in digest size paperbacks by Doubleday/Books, Inc. in the late 1950s, and the titles were again re-sequenced.From 1960 to 1964, eight titles were released by Berkley Medallion/Berkley Highland in paperback.