Sky Island

According to Wikipedia: "Sky Island: Being the Further Adventures of Trot and Cap'n Bill after Their Visit to the Sea Fairies is a children's fantasy novel written by L. Frank Baum, illustrated by John R. Neill, and published in 1912 by the Reilly & Britton Company — the same constellation of forces that produced the Oz books in the first decades of the twentieth century. 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)."