The Left Hand of Darkness - Ursula K. Le Guin

A lone human emissary journeys from Earth to the planet of Winter, an alien world whose inhabitants can choose - and change - their gender. His goal is to secure Winter's inclusion in a growing intergalactic civilisation, but to do so he must bridge the gulf between his own views and the culture he encounters. Forced into a perilous partnership with an outcast on a journey across the planet's bitter, whirling snow, he learns how survival, trust and love take shape in a world nothing like his own