The Royal Book of Oz

The Royal Book of Oz is the fifteenth in the series of Oz books, and the first by Ruth Plumly Thompson, to be written after L. Frank Baum's death. In this novel the Scarecrow discovers that, in a previous incarnation, he was human. To be precise, the Scarecrow was the King of the Silver Islands, a quasi-Chinese kingdom located underground beneath the Munchkin region of Oz. When Dorothy first discovered the Scarecrow he was hanging from a scarecrow-pole in a cornfield; it now develops that this pole descended deep underground to the Silver Islands, where it penetrated the king's grave.