Runes of the Lyre. Ardath Mayhar

Created by the powerful Hasyisi, yet missing for centuries from its home world, the Lyre just hangs in a willow tree, waiting. Hasyih, the Heart of the Worlds, links contiguous dimensions, many worlds invisible to each other, yet accessible through doors on Hasyih, one of the keys to which is the Lyre. Now danger threatens both Hasyih and Ranuit, the only inhabited worlds in the group, and when a young girl takes the Lyre from the willow tree, a set of interlinked activities is set into motion. Moving from world to world, going into the hands of the one who needs it most at the time, the Lyre reveals its nature as not only a Key, but also as a Weapon, an Enigma, an Answer, and a resolution, affecting both Hasyih and Ranuit. And the girl Queen Yisri is the center of it all.