Running the Asylum, Ascended Fanon: Readers send Piers Anthony enough suggestions in fan mail that Xanth novels now consist almost entirely of such material. Piers Anthony acknowledges these contributions in his chapter-long Author's Note at the end of every book.
The way the Dolph/Nada Naga/Electra love triangle was resolved (which hinges on the Exact Words that the Good Magician Humphrey said) was suggested by a fan; in the Author's Note for Isle of View, Piers Anthony said that he hadn't thought of that solution, and had even already written an ending in which the wrong girl "won".
Tribute to Fido: While Piers Anthony was writing Demons Don't Dream, his dog Bubbles died, so he decided to put her into the book at that point. Mare Imbri is also based on Blue, a horse he owned at the time. Blue died right before he started writing the same book, so, as he pointed out in the author's note, it seemed right that Mare Imbri would lead Bubbles into Xanth.