Glinda of Oz (Oz Series, Book 14)

Glinda of Oz is the fourteenth book in the Oz series written by children's author L. Frank Baum, published on July 10, 1920. It is the last book of the original Oz series, which, following Baum's death, was continued by other authors.
Princess Ozma and Dorothy travel to an obscure corner of the Land of Oz to prevent a war between two local powers, the Skeezers and the Flatheads. The leaders of the two tribes prove obstinate, and they are determined to fight in spite of Ozma and Dorothy. Unable to prevent the war, Dorothy and Ozma find themselves imprisoned on the Skeezers' glass-covered island, which has been magically submerged to the bottom of its lake. Their situation worsens when the warlike queen Coo-ee-oh, who is holding them captive and who alone knows how to raise the island back to the surface of the lake, loses her battle and gets transformed into a swan, forgetting all her magic in the process and leaving the inhabitants of the island, with Ozma and Dorothy, trapped at the bottom of the lake. Ozma and Dorothy summon Glinda, who, with help from several magicians and magical assistants, must find a way to raise the island to the surface of the lake again and liberate its inhabitants.