Left Hand of Darkness

This is the 1970 winning Hugo. In a lot of ways it is more an outdoor adventure story than science fiction.  There is the matter of the main character being a visitor from space to a planet populated by people who have no gender most of the time.  Whereby at the right time and place, a couple will change shape to accommodate being male or female this time around.  The visitor is trying to lead them to the light ie the Left Hand of Darkness is really the Light in Yin Yang terms.  A good read.