Glinda of Oz | L. Frank Baum

The last book in Baum's Oz series sees Princess Ozma and Dorothy travel to an obscure corner of the Land of Oz in order to prevent a war between the Skeezers and the Flatheads. The leaders of the two tribes are determined and Dorothy and Ozma find themselves imprisoned on the Skeezers' glass-covered island, which has been magically submerged to the bottom of its lake. Ozma and Dorothy summon Glinda, the most powerful sorceress of Oz, who, with help from several magicians and magical assistants, must find a way to raise the island and liberate its trapped inhabitants.