The Color of Her Panties - Piers Anthony

A merwoman and a goblin girl each take on dangerous quests in this hilarious fantasy adventure by a New York Times-bestselling author.
In Xanth, almost everything is magic, and whatever isn't is probably lying. The land is also more dangerous than the sea, but that isn't going to stop Mela Merwoman, who is searching for a husband. With her options running low, she transforms into a human and leaves the safety of the sea to ask Good Magician Humfrey for assistance. But before Mela can begin her quest, she must first indulge in the landbound custom of wearing clothes, which means picking out what to wear . . . from the trees, of course.
While Mela can pick her clothes, Gwenny Goblin cannot pick her family. Her awful half-brother, Gobble, will be the next chief of the goblin horde, if Gwenny doesn't take the title first. To do so, she must prove her courage through stealing an egg that lies between a deadly roc and a hard place. With the help of Che Centaur and Jenny Elf-and maybe even Mela-Gwenny just might make it back home alive . . .