Ursula Le Guin is the boss of science fiction. This and Dispossessed are my most prized science fiction books of all time. She is the master and all else pale. A good book has a great story line that is built as you read and uses the Greek heroic forms, but a great book teaches us things and leaves us a different person because of our experience of reading. The Left hand of Darkness is not a new book and others have written in a similar vein - see Glory Seasons from David Brin so if you have read them LHOD doesn't have the same impact but when it first came out it was startling.
I have been reading the LHOD for over 20 years and go back to it every few years. It does take a while to build so you have to get to page 80 before things become clear - there is a lot of world building to get through...
It is not relevant to offer a synopsis as the machinations are relatively unimportant - a bit like James Cameron's Avatar Pretend blue guy meets real blue woman and after a few bad patches becomes good friend - it is the alien people in an alien world that asks you to suspend your assumptions and learn a new way of dealing with the world. This is a book that gets you to explore who you are. It is a story write not so much with words but emotions, values and beliefs.
Have enjoyed books by Ursula Le Guin before but never read this one - -was prompted to by the reference to it in Jane Austen Book Club movie and found it an excellent read. Now reading her earlier trilogy set in the same universe.
This book is consistently rated in the top twenty SF books of all time. What if there were not fixed gender roles? Thats what it asks. Read this one and her Dispossed and you are reading some real human condition stuff. Give the space marine form of SF a rest and give your brain a workout with some beautiful prose.