Not a "series" in a narrative sense, this term describes twelve novels Heinlein published with Scribner between 1947 and 1958, typically released in the autumn in time for Christmas, and includes such works as "Citizen of the Galaxy", "Starman Jones", and "Red Planet". What term is typically applied to these works, which ended after Scribner rejected "Starship Troopers" as a 13th entry.
juveniles