The Green Sky Trilogy is a series of fantasy novels by Zilpha Keatley Snyder, originally published between 1975 and 1978 by Atheneum. The books feature illustrations by Alton Raible.The story takes place on an alien world called Green-sky (although the trilogy as a whole is referred to as the Green Sky Trilogy).The 1984 game Below the Root is a continuation of the trilogy’s plot. Unusual for a video game, its story is written by the author and is considered canon.