Series by Andre Norton
Excluding the World renowned series "Witch World", there are 26 separate series within the Worlds of Andre.
Whereas there are several series that are rather popular, there are probably 15 to 20 that most people are unaware of.
Ranging from Historical Novels, Romance, Young Adult, Space Opera, Fantasy and of course good-ol Science Fiction.
What exactly is a series ~ any group of 2 or more titles with enough in common that Andre said "these are a series".
Cycle of Oak, Ash, Yew and Rowen