The Left Hand of Darkness is the book I recommend most often to people who say they've never found a science fiction novel that felt relevant to their actual life. It is set on a world where humans have no fixed sex — they are periodically fertile in one sex or the other and otherwise sexless — and the book uses this premise to ask what human behavior is actually attributable to biology versus culture.

The protagonist Genly Ai, an envoy from a galactic civilization, keeps misreading every interaction on Gethen because he imports his own cultural assumptions about gender. The novel is largely about him slowly understanding how wrong those assumptions are. Le Guin is writing about how we all inhabit frameworks we can't see from inside them.

The prose is the opposite of flashy. It is measured, careful, and builds trust over 280 pages until the final section lands with a force that I still find surprising after three readings.