Montporte Dungeon campaign is particularly creative or unique. However, my sources of inspiration and influences might strike you as being a bit off the beaten path.
The Montporte Cosmological Appendix N
Baker, Keith, et. al.: Eberron Campaign Setting (Dungeons & Dragons 3.5)--I am not too proud to put this on the list. I do like the treatment of the planes in the Eberron setting, at least for 3.5e.
Butcher, Jim: "The Dresden Files"--I lifted my approach to the Fey from Butcher. Anyone can plainly see that fact. Demons, too, for that matter.
Copleston, Frederick: A History of Philosophy, Volume 1: Greece and Rome From the Pre-Socratics to Plotinus--I borrowed a bit from either end of the time frame represented in this book, the Pre-Socratics and Plotinus.
de Lint, Charles: "Newford Series"--A loosely connected series of urban fantasy novels and short stories that rely heavily on Celtic and Native American myths. De Lint is one of my favorite authors.
Dent, Greg: Oathbound: Domains of the Forge (D20)--This detailed setting for D&D 3.5e gave me all sorts of ideas.
Elert, Werner: The Structure of Lutheranism--I know this is a strange edition to the list, but Elert was a deep thinker and his reflection on the nature of the Creator, the creation, time and eternity is very profound and thought-provoking, even if one has a different view or belief. This is not for the faint of heart, however.
Greene, Brian: The Fabric of the Cosmos: Space, Time, and the Texture of Reality and The Elegant Universe: Superstrings, Hidden Dimensions, and the Quest for the Ultimate Theory--I am a big fan of Brian Greene. Check out his TED Talks: "Making Sense of String Theory" and "Is Our Universe the Only Universe?" He makes physics almost comprehensible.
Grossman, Lev: The Magicians and The Magician King--Sort of a Narnia for slackers and hipsters. I like his travel between worlds and I have borrowed some specific things from him. The players in the Montporte Campaign should disregard this entry on the list and pretend they didn't read this.
Jung, Karl: The Red Book (plus other works)--Just in case anyone wonders where the funky metal trees originated.
Koyre, Alexandre: From the Closed World to the Infinite Universe--This book is even more fun than the title might hint at. I read most of it in one sitting while eating meatloaf at a diner on Amsterdam Avenue in NYC. That seems about right.
Le Guin, Ursula: "Earthsea"--Ursusa Le Guin's Earthea books have probably had a greater impact than any other books over the course of my gaming life.
Moorcock, Michael: "Elric of Melniboné," "The Swords Trilogy"--Like
the gamers before and after me, Moorcock is a major inspiration.
However, as much as I love the Elric series, I have an even bigger crush
on the "The Swords Trilogy," featuring Corum Jhaelen Irsei.
Pratchett, Terry: Small Gods and the rest of the "Discworld" novels--Small Gods is my favorite Discworld novel and its approach to the gods have crept into my cosmology.
Rahner, Karl: Foundations of Christian Faith--This is heavy lifting. Rahner had a major theological impact on the Council of Vatican II and this book gives a hint as to the power of his intellect.
Suzuki, Shunryu: Zen Mind, Beginners Mind--I am not sure if I can explain how this book impacted my approach to gaming, but it has in some surprisingly profound ways.
Zelazny, Roger: "The Chronicles of Amber"--An awesome fantasy series with a cool approach to the universe and its multiple worlds. Thanks to Tim Shorts (Gothridge Manor) for recommending Zelanzy to me.