The Left Hand of Darkness
by Ursula K. Le Guin

Envoy Genly has come to the planet Winter to passively convince its inhabitants to join the interplantary union, the Ecumen.  The inhabitants of Winter are genderless and that's where the book gets very interesting.  The people of Winter have the physical traits and characteristics of both men and women, making them only identifiable as human rather than as a man or woman.  When a native Winterite enters their kemmer cycle, they can become either male or female.  A person who is father to two children may be mother to three others.

The genderless and sexual urge-less (aside from kemmer) society is very interesting.  Wars do not happen, but there are land disputes.  There is not the traditional labor divide one would find on Earth where we still have jobs thought of as more manly or feminine occupations (think construction workers and hairdressers).  On Winter, no such presupposition exists.  Any Winterite can do any task or job as well as any other, the only determining factor is individual ability.

I highly recommend this book for sci-fi fans.  I put it right up there with And Having Writ and The Moon is a Harsh Mistress.