Series

• Mythology
• Mythology 101 (1990)
• Mythology Abroad (1991)
• Higher Mythology (1993)
• Advanced Mythology (2001)

* Applied Mythology (2000) ; (An omnibus of books 1-3)

• MythAdventures
• Myth-told Tales (2003) with Robert Asprin
• Myth Alliances (2003) with Robert Asprin
• Myth-taken Identity (2004) with Robert Asprin
• Class Dis-Mythed (2005) with Robert Asprin
• Myth-Gotten Gains (2006) with Robert Asprin
• Myth-Chief (2008) with Robert Asprin
• Myth-Fortunes (2008) with Robert Asprin

• Dreamland
• Waking in Dreamland (1998)
• School of Light (1999)
• The Grand Tour (2000)

• Ireta (aka Dinosaur Planet, aka Planet Pirates)
• The Death of Sleep (1990) with Anne McCaffrey
• The Planet Pirates (1993) with Anne McCaffrey and Elizabeth Moon ; (omnibus edition includes The Death of Sleep)

• Doona
• Treaty at Doona (1994) with Anne McCaffrey
• Crisis on Doona (1992) with Anne McCaffrey

• Taylor's Ark
• Taylor's Ark (1993)
• Medicine Show (1994)
• The Lady and the Tiger (2003)

• The Ship Who Sang (aka Brain and Brawn Ship)
• The Ship Who Won (1994) with Anne McCaffrey
• The Ship Errant (1996)
• The Ship Who Saved the Worlds (2003) with Anne McCaffrey ; (includes The Ship who Won and The Ship Errant)

• Riverworld
• If the King Like Not the Comedy (1993)

• Crossroads Game Books
• Dragonharper (1987) ; set in the world of Pern
• The Encyclopedia of Xanth (1987) ; set in the world of Xanth
• Dragonfire (1988) ; set in the world of Pern
• Ghost of a Chance (1988) ; set in the world of Xanth

Novels

• The Magic Touch (1996)
• License Invoked (2001) with Robert Asprin
• Strong-Arm Tactics (2005)
• An Unexpected Apprentice (2007)
• A Forthcoming Wizard (2009)

Anthologies (As Editor)

• Don't Forget Your Spacesuit, Dear (1996) ; (includes the short story What's the Magic Word?)

Others

• The Dragonlover's Guide to Pern with Anne Mccaffrey et al.
• Piers Anthony's Visual Guide to Xanth (2000) with Piers Anthony, Todd Cameron Hamilton and James Clouse