My suggestion would be The Primal Order. It's an old book (the first published by WotC), but available on Drive Thru RPG.
The Primal OrderTM reigns supreme. Also known as TPO, The Primal Order
is the premiere resource for using religions and deities effectively
and creatively in roleplaying games. Explore how deities gain power,
learn why they need worshippers and temples, discover why they provide
their priests with spells, and much more. TPO also offers detailed
guidelines for the creation of artifacts, suggesting strategies for
their design and use.
Heralded since its initial release as one of the most innovative and
useful roleplaying supplements ever published, TPO is an invaluable
tool for campaign development. Get ready—your roleplaying experience
is about to be transformed!
I used it with great effect in more than one game to
- allow characters to cross that threshold from mortality to godhood
- play a campaign that the characters started as gods
- create religions and such for campaigns
- stat up gods based not on the rules of the game, but rules that made them seem like gods
I highly recommend it, and am glad that it's available again. There were three other books also- Pawns, The Opening Move, Chessboards, The Planes of Possibility, and Knights, Strategies in Motion. I recommend the latter two more than the first.