Quite a few, here’s a list:
Xanth
1. A Spell for Chameleon (197
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2. The Source of Magic (1979)
3. Castle Roogna (1979)
4. Centaur Aisle (1981)
5. Ogre, Ogre (1982)
6. Night Mare (1982)
7. Dragon on a Pedestal (1983)
8. Crewel Lye: A Caustic Yarn (1984)
9. Golem in the Gears (1986)
10. Vale of the Vole (1987)
11. Heaven Cent (198
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12. Man from Mundania (1989)
13. Isle of View (1990)
14. Question Quest (1991)
15. The Color of Her Panties (1992)
16. Demons Don't Dream (1993)
17. Harpy Thyme (1993)
18. Geis of the Gargoyle (1994)
19. Roc and a Hard Place (1995)
20. Yon Ill Wind (1996)
21. Faun and Games (1997)
22. Zombie Lover (199
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23. Xone of Contention (1999)
24. The Dastard (2000)
25. Swell Foop (2001)
26. Up in a Heaval (2002)
27. Cube Route (2003)
28. Currant Events (2004)
29. Pet Peeve (2005)
30. Stork Naked (2006)
31. Air Apparent (2007)
32. Two to the Fifth (200
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33. Jumper Cable (2009)
34. Knot Gneiss (2010)
35. Well-Tempered Clavicle (2011)
36. Luck of the Draw (2012)
37. Esrever Doom (2013)
38. Board Stiff (2013)
Spellbound Xanth (omnibus) (1986)
The Magic of Xanth (omnibus) (1987)
More Magic of Xanth (omnibus) (1989)
Piers Anthony's Visual Guide to Xanth (1989) (with Jody Lynn Nye)
A Spell for Chameleon / The Source of Magic / Castle Roogna (omnibus) (1995)
The Continuing Xanth Saga (omnibus) (1997)
Xanth: The Quest for Magic (omnibus) (2002)