Horrors of the Dancing Gods — Jack L. Chalker

MORTAL DILEMMA
An ancient evil was seeping forth from the Sea of Dreams: the Old Ones were rising from the depths to challenge both Heaven and Hell. Their assault boded ill for Earth, and worse for the magical land of Husaquahr, where magic was played by the Book. Under the Rules, only the Great McGuffin could stop that looming menace.
But the McGuffin was lost, somewhere in Hell....
Master wizard Throckmorton P. Ruddygore once again summoned a handful of heroes to stave off disaster. But Joe, the truck-driver-barbarian-turned-wood-nymph, was busy with his--or her?--own problems, so Ruddygore had to assign Marge, the changeling nymph, to shepherd an untried youngster on his first quest. Her new protégé was none other than Irving, Joe's human son: estranged, bitter--and irresistibly attractive!
Marge wanted to turn down the mission, but too much depended on it! Once in the dark realm, though, her good intentions began to erode. Could any nymph just say "no" to a son of Joe?