The Left Hand of Darkness, by Ursula Le Guin

This is one of the most enjoyable novels I've read in some time.  The writing is beautiful - sparing and descriptive.  There is an atmospheric quality that suffuses the narrative but, unlike the planet's snow and ice, doesn't smother it.  The setting is a wintry planet populated by humans who are neither fully male nor female most of the time (hermaphroditic may be the best way to think about it) who monthly for a period of a couple of days change into either fully male or female form so they can reproduce.  The rest of the time they are androgenous without sexual drive.  The implication of this for their society and their relationships is the real gem in this story.  Truly a great read.