L. Frank Baum 
Glinda of Oz [EPUB ebook] 

This is the fourteenth and last Oz book. Princess Ozma and Dorothy attempt, unsuccessfully, to stop a war between two tribes, the Skeezers and the Flatheads. The two of them are imprisoned on a glass-covered island belonging to the Skeezers, which is now located at the bottom of a lake. Only their queen, Coo-ee-oh, can raise the submerged island but after losing in battle, she becomes a swan and forfeits her magical abilities. Glinda ultimately comes to Ozma and Dorothys rescue.