Ruth Plumly Thompson 
The Illustrated Royal Book of Oz [EPUB ebook] 

In this rousing OZ adventure Scarecrow goes off in search of his past. Dorothy and the Cowardly Lion search for him, eventually meeting with a knight, Sir Hokus, the Doubtful Dromedary and the Comfortable Camel. Scarecrow discovers that, in a previous incarnation, he was human! He had been the King of the Silver Islands, a quasi-Chinese kingdom located underground beneath the Munchkin region of Oz. This fully illustrated edition contains nearly 100 of the superb original illustrations by John R. Neill.