The Left Hand of Darkness

Genly Ai is sent to the frozen planet Gethen to persuade its nations to join an interstellar alliance. He finds a society whose inhabitants are neither male nor female for most of the month, and an ally, Estraven, who is both a friend and a political danger.

Le Guin uses the setting as a thought experiment, asking how much of what we call human nature is actually culture. The ideas are bold, yet the heart of the book is the friendship between two men from different worlds, tested by an arduous trek across the ice.

The pacing is quiet and the invented names take some patience. In return you get one of the most humane science fiction novels ever written.