Below the Root — Zilpha Keatley Snyder

Raamo, at thirteen, had rarely doubted the wisdom of the Ol-zhaan, the unquestioned rulers of the Green-Sky planet. Yet, after he had been chosen to become an Ol-zhaan, he made surprising discoveries and was exposed to dangers different form any he had envisioned. The world of Green-Sky was not what he and the Kindar people had thought. This engrossing science fiction fantasy was first published by Atheneum in 1978 and is the first book of the Green-Sky Trilogy.