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.