Swell Foop

The twenty-fifth book of Xanth deals with a matter of some gravity. Specifically, the Demon Earth has disappeared, which means that eventually not only some but all gravity will disappear from Earth and Xanth. To solve the mystery and rescue the Demon Earth, the Swell Foop is required. That involves using the six magical rings of Xanth, which need to be found by means of a quest by five old friends of Xanth mavens - Cynthia and Che Centaur, Brianna and Justin Tree, and Sim, the immortal bird - as well as Jaylin, a girl from Mundane Hawaii.