The Royal Book of Oz
 In which the Scarecrow goes to search for his family tree and discovers that he is the Long Lost Emperor of the Silver Island

by Ruth Plumly Thompson

"The Royal Book of Oz" by Ruth Plumly Thompson and L. Frank Baum is the fifteenth Oz book, published in 1921. When the Scarecrow ventures back to the cornfield where Dorothy first found him to discover his origins, he disappears without a trace. Dorothy and the Cowardly Lion embark on a search mission, joined by an elderly knight and unusual companions. Their quest leads to an astonishing revelation: the Scarecrow was once human, an emperor of an underground kingdom with a mysterious past.