Glinda of Oz

In the final book in the original Land of Oz series, Glinda the Good must save Dorothy and Princess Ozma from a perilous underwater island.

When the peace and prosperity of Oz is threatened as the Flatheads and the Skeezers—two obscure tribes who inhabit a far corner of the realm—declare war on each other, Dorothy Gale and Princess Ozma embark on a desperate journey to put an end to the conflict. But with both sides determined to fight, Dorothy and the princess soon find themselves held captive on the Skeezers' island, which is covered in glass and submerged at the bottom of a lake.
The Skeezer queen, Coo-ee-oh, is the only one who knows the charm to bring the island back to the surface. But when she herself is put under a spell, it's up to Glinda the Good to save the day—with a little help from the Wizard of Oz, the Scarecrow, the Cowardly Lion, and the Tin Woodman.