A highly imaginative, original, and underrated, world setting.
Also had the distinction of having a sequel in the form of a video game, with the game's story written by the book author herself. [1]
The game (for the PC, Apple II and Commodore 64) was way ahead of its time in 1984: [2] and I only just heard of it and the books last month! It definitely needs more recognition.
[0] https://en.m.wikipedia.org/wiki/Green_Sky_Trilogy
[1] http://blog.stahlmandesign.com/below-the-root-a-story-a-comp...