The Trillum books were created by Marion Zimmer Bradley. Andrea Norton and Julian May, each writing the part of one sister all princess.

The Trillium series comprises:
Black Trillium (written by MZB, AN and JM)
Blood Trillium (written by JM)
Gold Trillium (written by AN)
Sky Trillium (written by JM)

The story is pure fantasy - without the dragons - set in a world where technology has fallen and magic has risen, each book follows both it's own arc and that of the larger story, the story of how these sisters fair against the odds to bring piece beneath the three moons.