I'm surprised no one mentioned this classic yet: The Left Hand of Darkness by Ursula K. Le Guin

A lone human ambassador is sent to the icebound planet of Winter, a world without sexual prejudice, where the inhabitants’ gender is fluid. His goal is to facilitate Winter’s inclusion in a growing intergalactic civilization. But to do so he must bridge the gulf between his own views and those of the strange, intriguing culture he encounters.