The Left Hand of Darkness by Ursula K. LeGuin

When the human ambassador Genly Ai is sent to Gethen, the planet known as Winter by those outsiders who have experienced its arctic climate, he thinks that his mission will be a standard one of making peace between warring factions. Instead the ambassador finds himself wildly unprepared. For Gethen is inhabited by a society with a rich, ancient culture full of strange beauty and deadly intrigue—a society of people who are both male and female in one, and neither. This lack of fixed gender, and the resulting lack of gender-based discrimination, is the very cornerstone of Gethen life. But Genly is all too human. Unless he can overcome his ingrained prejudices about the significance of "male" and "female," he may destroy both his mission and himself.