The Left Hand of Darkness (Hainish Cycle #4)
by Ursula K. Le Guin

See alternate cover editions here and here.

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

The Ekumen of Known Worlds has sent an ethnologist to study the Gethenians on their 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