Ninefox dumps you right into the action, and it is a trip because from the beginning it is apparent that the rules of things like reality work very differently in this universe, and things are never really explained, so much as left to the reader to work out. I enjoy working out these things out, so Imma spoiler-cut these details, but in this universe, math is not so much descriptive, as effective, that is, doing math produces an effect. And doing math does not mean doing calculations, but rather doing math as a cultural practice, things like agreeing on a number system, and the geometry of streets. The Hexarchate maintains its military power by imposing a calendar system on its peoples, and where that calendar is practiced, the Hexarchate's military weapons work. Rebels begin by practicing calendrical heresy, destabilizing the Hexarchate's terrain.

The area of space in which the action takes place is ruled by the Hexarchate, a kind of terrible governing body. Each hexarch rules one of six factions, and faction membership is not necessarily hereditary. The Kel faction is the foot-soldiers' faction, and the Kel virtue is loyalty: unquestioning, and involuntary. Cheris is a Kel Captain, dangerously free-thinking by Kel standards because she sometimes does her own math.

Then Cheris is called up to deal with heretics, and given one of the Hexarchate's most effective and little-understood weapons; the ghost of mad, undead general, anchored to the living world by her mind. The general turns out to be more or less as advertised, and not a great roommate.

You guys, I really love this book and think everyone should read it. If you've been put off because I mentioned math, don't worry, there's no actual math contained in the text. It's alien in a way that real cultures are and few fictional cultures manage to be. Cheris and Jedao, the ghost-general, have a great dynamic, and it's one of my favourites, which is "I can't trust this person, but for now, we have to work together."