The left hand of darkness - Ursula K Le Guin

Winter is an Earth-like planet with two major differences: conditions are semi artic 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.