Glinda of Oz - Lyman Frank Baum

The fourteenth book in L. Frank Baum's Land of Oz series is titled Glinda of Oz. To stop a conflict between the Skeezers and the Flatheads, two local powers, Princess Ozma and Dorothy, journey to a remote area of the Land of Oz. In spite of Ozma and Dorothy, the two tribes' chiefs remain adamant and ready to war. Dorothy and Ozma discover themselves imprisoned on the Skeezers' glass-covered island, which has been magically sunk to the bottom of its lake, unable to stop the conflict. The warlike queen Coo-ee-oh, who is keeping them captive and the only one who knows how to raise the island back to the surface of the lake, loses the battle and transforms into a swan, forgetting all her magic in the process, trapping the inhabitants of the island, including Ozma and Dorothy, at the bottom of the lake. This makes their situation worse. Glinda is called upon by Ozma and Dorothy. With the aid of numerous magicians and magical helpers, she must discover a means to lift the island back to the lake's surface and free its residents.