For gaming-oriented sources to help you generate ideas, we'd be horribly remiss in not mentioning
GURPS Religion. That book was a great help to me with both flavor and mechanics when I was fleshing out my homebrew fantasy GURPS campaign.
Green Ronin's
The Book of the Righteous gives a multitude of worked examples of fantasy religions, and a chapter with advice on how to do it yourself. (The original was for D&D 3.0, but GR recently Kickstartered an update to 5E. The PDF is available in their online store, and the print version should reach backers and stores soon.)
For the past decade or so, I've been running a Greek myth-based solo BESM campaign for my wife. One of the best sources I've found for ideas for that is Robert Graves'
The Greek Myths. As
recently discussed on the
Ken and Robin Talk About Stuff podcast, Graves is valuable both for his concise retellings of the myths (including alternate versions of many stories) and for the crazypants but eminently gameable theories that litter his footnotes.