The Dispossessed

This novel, like many of LeGuins other novels, is a sociological/philosophical work of science fiction. The story is about two completely different societies symbolized by a planet and its populated moon. The citizens living on the planet live in a capitalistic economy while the denizens of its moon live in a communist/anarchist society. The two worlds have no contact with another until the physicist Shevek attempts to break his moon-world's isolation.
This is a very good book with a lot of thought-provoking ideas and insights.