Piers Anthony & Piers A. Jacob 
Xanth 15: The Color of Her Panties [EPUB ebook] 

We See England, We See Xanth, We See Mels’s Underpanth In fact, Gwenny Goblin, Che Centaur and Jenny Elf are just about the only creatures on Xanth who have been spared the sight of Mela Merwoman’s undergarment — preoccupied as they are with helping Gwenny beat out her awful half-brother Gobble for chiefship of the goblin horde. But first they must master space and thyme . . . and find the fabulous egg that sits between the Roc and the hard place. While Mela — who would gladly relinquish her oft-viewed undies for a new husband — joins the Adult Conspiracy . . . and quickly discovers the power of a perfect pair of panties!