The Royal Book of Oz [Wizard of Oz #15]

In THE ROYAL BOOK OF OZ, the Scarecrow slides down a magic bean-pole and discovers he is actually the Emperor of the mysterious underground Silver Islands. When Dorothy discovers him missing, she sets out to find him, meeting the knight, Sir Hokus of Pokes, along the way.

Ruth Plumly Thompson (1891-1976) was an American author of children's stories.  Thompson was already an established children's writer when she was approached to continue the Wizard of Oz series following L. Frank Baum's death in 1919.  Between 1921 and 1939, Thompson published a new Oz book yearly.  After a more than thirty-year lapse, she published two new Oz stories in the 1970s, just prior to her death.