The Left Hand Of Darkness by Ursula K. Le Guin
Winter is an Earth-like planet with two major differences: conditions are semi arctic even at the warmest time of the year, and the inhabitants are all of the same sex. Tucked away in a remote corner of the universe, they have no knowledge of space travel or of life beyond their own world. And when a strange envoy from space brings news of a vast coalition of planets which they are invited to join, he is met with fear, mistrust and disbelief. . . 'The Left Hand of Darkness' is a groundbreaking work of feminist science fiction, an imaginative masterpiece which poses challenging questions about sexuality, sexism and the organisation of society.