Vale of the Vole (Xanth)

When Esk, a young ogre-nymph-human, began his pilgrimage to the Good Magician Humfrey to rid himself of a seductive demoness, little did he know it would become a mission of mercy. A running river paradise and its harmless inhabitants were perishing in the wrathful wake of a greedy demon horde. Now it is up to Esk and his companions--a beautiful winged centaur named Chex and a brave burrower Called Volney--to search Xanth's treacherous reaches, gathering together a mind-boggling company of creatures to defend the precious Vale of the Vole.