L. Frank Baum & Ruth Plumly Thompson 
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. The Scarecrow goes back to the corn-field where Dorothy Gale found him to trace his "roots." When he fails to return, Dorothy and the Cowardly Lion set out to search for him.