Between the Rivers (Tor, 1998) is a fantasy novel by Harry Turtledove about a land whose culture is broadly based on Bronze Age Mesopotamia, and whose cities and regions are ruled by their own gods. In the city of Gibil, however, the god Engibil has become lazy and does not monitor his city, allowing the inhabitants to develop technology, such as bronze. Their respect for Engibil and his power has steadily eroded as they've grown more independent. The other gods have become angry at Gibil for an unknown reason, and now refuse to trade with Gibil.
A merchant, Sharur, must travel to find out why the gods are angry and try to solve the problem. Along the way, he encounters friends and enemies and is always trying to convince people that the gods are unnecessary.
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"Bluff," a short story set on an alien planet with a quasi-Mesopotamian society where people frequently hear the voices of gods and ancestors in their heads. It is presented as science fiction rather than fantasy. Turtledove has stated that the ghost concepts in both "Bluff" and Between are based on the theories of Julian Jaynes. However, the ghosts and gods of Between the Rivers are real in that world.