Xanth #11 Heaven Cent Piers Anthony(1988)

In the mind of Xanth’s precious shapeshifting Prince Dolph, the perfect was to see the world is to search for the missing sorcerer, Humfrey. Setting off with his faithfuls companion, Marrow, an enchanted skeleton, Dolph will penetrate an island of illusion, escape a goblin kingdom, outwit a husband-hungry mermaid, save marrow from bone-starved harpies, and find romance with a slinky snake princess–all on his way to discovering a magic coin with all the answers!