Here, from my shelves to you, are five more works of nonfiction that I've find inspirational, or instructional, in the process of world-building:
Imaginary Worlds by Lin Carter: This first selection is an oldie--woefully out of print--but a goodie and worth seeking out. Not only does Carter provide a history of "secondary world" or "imaginary world" fantasy, but his last chapter is a "how-to" on world-building covering topics like religion, cartography, and naming. It's aimed at fiction writers, true, but it has some good thoughts for gaming world-builders, as well.
The Secret Teachings of All Ages by Manly P. Hall: Originally published in 1928, Hall's book is a good corrective to the simplified polytheistism in a lot of fantasy game worlds. He's got quick-read-but-detailed, chapter's on Pythagorean mysticism, Gnosticism, Hermeticism, and more mystery cults than you can shake a bone rattle at. It would probably be useful to add some color to modern, or modernish, occult games like Call of Cthulhu, too.
Dictionary of Ancient Deities by Patricia Turner & Charles Russell Coulter: Need inspiration for the portfolios or characteristics of gods in your game? Or maybe just need an obscure name to throw on an idol in a dungeon, and don't feel particularly like coining one? This books got you covered from A (Mayan death god) to Zywie (alternate name for Polish goddess of life, Ziva). It makes for interesting browsing for ideas you didn't know you needed, as well.
Encyclopedia of Heresies and Heretics by Chas S. Clifton: What's all this Ancient World-syle religion in a pseudo-medieval setting? Try out some of these challenges to Christian orthoxy through the ages. I've found heretical beliefs a big inspiration for "Catholicism-but-not" religions for games that need something like that. Not as much information as Cohn's book I've mentioned previously, but a breezier read and more browsable.
Monsters! by Neil Arnold: Arnold subtitles his book "the A-Z of zooform phenomena" giving a hint of his Fortean stance, but its weird sightings, urban legends, and mythological creatures make for fine adventure fodder. Arnold's entries suffer from a little sparseness of detail at times, and some of the monsters either already have analogs in gaming or would require a lot of thought to make them useful. But things like the Hopkinsville Goblins, Jenny Greenteeth, or the cattle-mutilating, flying, Jellyfish of Japan, definitely have potential.
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