The Left Hand of Darkness (1969)
by Ursula Le Guin
I really enjoy all the books set in Le Guin's Hainish universe - a universe in the far future, where many planets spread throughout space have been populated by humans, although I'm not quite sure how they got there because that's in the distant past when the books take place. Each world has its own unique climate, history, culture. The books are frequently told from the view point of an outsider who has come to study (like an anthropologist) the various cultures that have developed on each world.
The Left Hand of Darkness is truly a classic and is the winner of both the Hugo and the Nebula awards. The book is set on the planet Winter and its people are biologically hermaphroditic. They can be either male or female once a month when they "go into heat" (for lack of a better term) and either party can become pregnant and bear children. This allows Le Guin to write a fascinating exploration of gender and how the lack of such a basic duality (or so our culture thinks, anyway) could create a different mindset entirely. Unfortunately to my mind (and also Le Guin's, I believe, I seem to recall reading a recently-published essay where she agreed with my point), the characters are referred to as "he" even though they are not male, which makes it hard for reader to view the characters as non-gendered.
I read this book many years ago and enjoyed re-reading it recently. I had forgotten the plot of the book, which was political intrigue, and the interspersing of myths and folklore from Winter in the story line that helped flesh out the cultures.
I also re-read the short story Coming of Age in Karhide from Le Guin's short story collection Birthday of the World. It is set in the years following Left Hand, and told from the point of view of a person from Winter, so was a nice addition. In this case, Le Guin used more gender-neutral words, which I preferred for getting the point across.