Best known for the Oz stories, beginning with The Wonderful Wizard of Oz, L. Frank Baum wrote a variety of other works, including plays and newspaper stories. Sky Island is the sequel to Baum's The Sea Fairies. Trot is the young daughter of a California schooner captain. She is accompanied by Captain Bill -- an old sailor with a wooden leg who was her father's captain.
Trot meets Button-Bright, a boy using a magic umbrella to travel from his home in Philadelphia. Trot, Button, and Captain Bill decide to travel with the umbrella, which takes them to a literal island in the sky. Sky Island is divided in half, one side pink, and the other blue, and the blues and pinks are at war.
Trot, Button, and Captain Bill are imprisoned, and must somehow escape and end the conflict so they can return home.