This book won the best novel Hugo award in 1981.
It's certainly science fiction, and discusses a number of scientific points in the setting. At the same time, the style of the writing and story is kind of like a fantasy novel, and according to some sources, the novel was in fact influenced by the fairy tale of the same title by Hans Christian Andersen.
The story starts on a planet called Tiamat. This planet's culture is divided into two main groups, Summers and Winters. The planet's orbit causes dramatic changes in climate to occur every 150 years, and when that happens, the Summers and Winters trade places as the ruling group, so they take turns. A queen rules the world, so there is always either a Summer Queen or a Snow Queen.
The Snow Queen, Arienrhod has some of her genetic material used to impregnate a number of Summer women. By doing so, she hopes to orchestrate the rise to power of a Summer Queen who will be her clone, not just physically but also in spirit. This is, however, done in secret, since it would be considered unethical even for the Snow Queen.
The story follows Moon, a girl, and her cousin Sparks, a boy, both Summers. The Summers are less interested in technology and progress than the Winters, and tend to live in a way more based on old traditions. Moon and Sparks are lovers. But, when Moon starts training to be a sybil, it separates her from Sparks. A sybil is someone who can enter a trance-like state that allows them to access information accumulated throughout the galaxy, and answer questions.
Sparks goes to Carbuncle, Tiamat's capital, where he ends up being found by Arienhrod - not entirely by accident - and becomes the "Starbuck," the Queen's lover. He first has to fight the old Starbuck, and win. He also has to lead the hunt for mers, creatures whose blood hold a chemical that can hold off the effects of growing old and extend life for decades.
Moon, not knowing that she is a clone of Arienrhod, finds out more about how Tiamat is being manipulated by other worlds in the galactic Hegemony. She realizes this has harmed Tiamat and wants to change it.
One thing I liked about this novel was the prose, I found it very vivid, and it made me feel like I was right there with the characters, seeing and hearing and feeling everything they did.
I found it fascinating to see how of the two characters, Moon and the Snow Queen turn out to be very different people, despite Moon being a genetic clone of Arienrhod. Raised in the summer culture, Moon has different values and does not think the same way as Arienrhod. Maybe Vinge was trying to venture an opinion on the the nature versus nurture debate, a question that's never been definitively answered.
Arienrhod seems to come across as far more selfish and manipulative. Moon seems like a person more sincerely interested in helping others and doing what she can for the greater good of everyone.
Although, there's an ironic twist in what does happen to Moon in the end - spoiler warning - just as Arienrhod wanted, Moon does become the Summer Queen. So despite their differences and Airenrhod's realization that Moon isn't the same as her, part of Airenrhod's plan does come to pass.
I also noticed how much of the behavior of the characters was defined by the setting and culture, to the point where some people are expected to make considerable sacrifices in the name of their culture. This is especially true of people who work in government, which is ironic as well because such people are often perceived as having a lot of power. But in this book, many of them come across as being very bound by customs and rules, and what the public perception of their actions will be.
This novel certainly has some strong themes, but they're explored subtly, which makes it a good read. Intelligent and thoughtful, with good characters. I found the pace to be a little bit slow, which isn't a criticism - but if you prefer more intense, fast-paced material, this might not be your favorite type of book.
There were a few things that made it seemed little dated - some things about the characters' attitudes and interactions seemed like they were influenced by the late seventies type of thinking, but not too much.
All in all, a good read.