Small Gods, a discworld book, is set in Omnia, which is in the Klatchian desert, and in Ephebe, which borders the desert and the Circle Sea. On the Disc, gods need believers - not just worshipers, but actual believers. They don't need them to survive, exactly, but the more believers a god has, the more powerful he, she or it is. Some gods are very small gods indeed, like Om, who happens to be (at the moment) a tortoise. He's doing better than some, though; he's got a whole city of violent, expansionist worshipers, and exactly one believer.
Which is why he's a tortoise.
Small Gods is in Pratchett's usual satirical style, poking fun at religion and helping us see it from another angle. Unfortunately, this book doesn't sparkle like some of the others I've read. It's alright, and it has moments of humor (see below for some quotes), but it didn't have me laughing out loud. I wish I could point to a defect and say, "here's the problem," but I can't.
If you like Pratchett, you'll enjoy this one, but if you've never read him before, start somewhere else!
Cuius testiculos habes, habeas cardia et cerebellum.
"...That's why it's always worth having a few philosophers around the place. One minute it's all Is Truth Beauty and Is Beauty Truth, and Does a Falling Tree in the Forest Make a Sound if There's No One There to Hear It, and then just when you think they're going to start dribbling one of 'em says, Incidentally, putting a thirty-foot parabolic reflector on a high place to shoot the rays of the sun at an enemy's ships would be a very interesting demonstration of optical principles."
XIX. Hah! You Want A Constitutional Religion?
"Why not? The other sort didn't work."
VI. This Is Religion, Boy. Not Comparison Bloody Shopping! You Shall Not Subject Your God To Market Forces!