I have no idea how to rate this book. I can see perfectly well that it's a thoughtful, intelligent, well-written book (it is a sci-fi classic, after all), with some fairly profound themes running through it. It's a book that rewards careful reading, rather than being a dramatic race to the finish. The characters and background are beautifully drawn. And yet... somehow it's not engaging. It reads like an intellectual exercise, a very literate one, to be sure, but still, it felt to me as though the author's aim was to examine certain issues rather than simply telling a story. It was interesting, and enjoyable in its way, but not emotionally engaging.
I first read this around 1980, and it made enough impression on me that some images were etched indelibly on my brain. Basic premise: Gethen is a world of humans genetically modified at some point in the past to be hermaphrodite, only taking male or female physicality during a few days each month ('kemmer', like being in heat); Genly Ai is an envoy who arrives from an interplanetary confederation to invite the Gethenians to join their fellow humans in a loosely organised trading network of equals. It's an easy read, not too steeped in obscure terminology, and a lot of the background is explained in extracts from historical stories or recordings scattered throughout, or journal entries, or through the comments of Genly, the outsider. And, unlike most sci-fi, there is no advanced technology to deal with, no aliens, no squids in space - just humans with relatively undeveloped technology and recognisable social structures (specifically, a kingdom and a bureaucracy).
Apart from Genly Ai, the envoy, the other main character is Estraven, the king's senior minister of state, initially, and Genly's only friend. The fragile relationship between the two states, the kingdom and the bureaucracy, is a fundamental part of the plot, and the political machinations cause trouble for both Genly and Estraven, but the essence of the story is the question of trust, and how gradually all pretensions and evasions between the two protagonists are stripped away, leading them to a better understanding of each other.
The writing is gently literate, and becomes very philosophical at times. It would be possible to write a doctorate level thesis on some of the themes invoked - the duality of being, the nature of belonging and love and brotherhood, the pride of the individual or the state. There are the two wildly different political systems, and the book touches on religion and 'mind speech'. And there is Gethen (or 'Winter') itself, which is more than just a cold backdrop for the story, it actually becomes the story towards the end.
Ultimately, the book is enjoyable without ever becoming compelling. I liked the moments which illuminate the deep backstory (the origins of these not-quite-like-us humans on a planet with no other mammals), and there is much here that is very thought-provoking, but, while beautifully written, it is almost too cerebral to be emotionally engaging. The ending is a triumph, however, with Genly, the outsider who has been gradually and unknowingly assimilated, watching the arrival of his fellow humans, men and women, and seeing them as quite alien. A worthwhile read. Four stars.