The Ruby Key (ISBN 978-0545000130) is a 2008 fantasy novel by Holly Lisle.
Plot
editGennadara and Danrith are a brother and sister living in the village of Hillrush. Their father presumed dead, they are raised by their mother and uncle Bartis. Hillrush is surrounded by beings known as nightlings at night, so humans only go out during the day. One night, Gennadara and Danrith sneak to get taandu essence - the magical curing sap of the taandu tree - to cure their sick mother. They encounter nightlings, and are set on the path of adventure.