The Left Hand of Darkness — Ursula K. Le Guin

On the planet Winter, there is no gender. The Gethenians can become male or female during each mating cycle, and this is something that other cultures find incomprehensible.

The Ekumen of Known Worlds has sent an ethnologist to study the inhabitants of this forbidding, ice-bound world. At first he finds his subjects difficult and off- putting, with their elaborate social systems and alien minds. But in the course of a long journey across the ice he reaches an understanding with one of the Gethenians it might even be a kind of love...