Centaur Aisle Xanth Series, Book 4.

The magic of Xanth was useless in Muldania-until Dor tried honesty! Dor was having troubles growing up to be the next Magician-King of the magic Land of Xanth. He wanted no part of running the Kingdom. But now the Good King Trent was leaving on a trade mission to non-magical Mundania, home of such weird beasts as horses and bears, so Dor had to take over as King for a week. A week passes. No Trent. Then three weeks. King Trent still hasn't retur...