Castle Roogna

Millie, a ghost for 800 years wants only one man--Jonathan, and he's a zombie. To prove himself, Magician Dor volunteers to get the potion that can restore Jonathan to full life. But he has to go back through time to do it, to a peril-haunted, ancient Xanth, where danger lurks at every turn....

This is #3 in Xnath Series.