The Kingmaker — Nancy Springer

The daughter of the High King has a regal name, but she is small and plain, so everyone just calls her Wren. As a mere girl, she is not her father's heir; her cousin Korbye is. But Wren's infallible sooth-sense tells her that Korbyn would make no good king. Nor is sooth-sense her only fate. Wren is the Kingmaker. When an ancient and dangerous ring of power finds its way to her, how should she use it?