Sky Island - being the further exciting adventures of Trot and Cap'n Bill

As the full title indicates, Sky Island is a sequel to Baum's The Sea Fairies of 1911. Both books were intended as parts of a projected long-running fantasy series to replace the Oz books. Given the relatively tepid reception of the first book in the series, however, Baum tried to attract young readers by including two characters from his Oz mythos in Sky Island — Button Bright and Polychrome, originally introduced in The Road to Oz (1909).

Trot is near her home on the coast of southern California when she meets a strange little boy with a large umbrella. Button Bright has been using his family's magic umbrella to take long-range journeys from his Philadelphia home, and has gotten as far as California. After an explanation of how the magic umbrella works, the two children, joined by Cap'n Bill, decide to take a trip to a nearby island; they call it "Sky island," because it looks like it's "halfway in the sky" — but the umbrella takes them to a different place entirely, a literal island in the sky.