The Left Hand of Darkness, 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...

Embracing the aspects of psychology, society, and human emotion on an alien world, The Left Hand of Darkness stands as a landmark achievement in the annals of intellectual science fiction.