The Left Hand of Darkness by Ursula K. Le Guin
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Like new and unread.
A groundbreaking work of science fiction, The Left Hand of Darkness tells the story of a lone human emissary to Winter, an alien world whose inhabitants can choose -and change - their gender. 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 completely dissimilar culture that he encounters.