Although the cover of the book and many of the reviews classify this novel as "science fiction", I really consider it to be fantasy. Certainly, it has a lot of the trappings of a traditional SF space opera. It takes place in what appears to be a distant future of the human race when giant space battle cruisers equipped with powerful weapons and artificially intelligent robot servants battle for control of space stations. However, it is also a world in which a person's shadow and reflection can change to reflect their rank, or the "ghosts" that are linked to them. It is a world where eating the glass shard remains of someone's essence can give you their memories. And, it is a world where the ritual calendar (literally the holidays of the religious beliefs of a culture) affect the laws of physics themselves (which is why "heretics" who have their own calendar are so dangerous).

Because the weapons and battle formations depend in an unspecified mathematical way on the ritual calendar, Cheris has to be able to quickly do computations and informs her troops of their plans by showing them equations. It is her mathematical skills, rare in someone of the warrior class of her society, that lead her to be chosen for the dangerous mission which is the central plot of the novel. Mathematical terminology and discussions of mathematical ability show up frequently in the book. Indeed, it becomes clear to the reader that math is a very important skill in this world where it seems to be a sort of connection between science and magic, between technology and religion. The famous inventor of the "mothdrive", a mysteriously old but powerful figure, is said to not care about people "unless the people could keep up with him on things like number theory". Nevertheless, the reader does not need to know anything about math to understand the book and will not learn any math from it either.

In an interview about one of his earlier stories, Yoon Ha Lee said "I became fascinated by the idea that you could use mathematical imagery to inform a story. (I admit I'm a little scared of what a real mathematician would make of the story!)" Indeed, this novel too is filled with mathematical imagery. Some good examples are:

Lee needn't have worried about what a mathematician would think of it. Although it is true that I appreciate it when an author can work some real mathematical meaning into a work of fiction, this sort of impressionistic mathematics that generates some feeling without actually saying anything can also be good. Lee seems to do it as well as anyone else and better than some who do not even know how mathematicians use the same terms. For example, when he calls weapons that do not depend on the choice of calendar "invariant", it sounds mathematically correct.

I think the feeling of confusion of the strange game that is taking place is really what this book is about. There are many different classes of society, there is the hexarchate and their enemies the heretics, there are individuals we get to know and huge numbers of anonymous individuals who die in battles, and there is some complicated plan. Throughout the book, we don't quite really know what is going on, but we know that there is a competition, that the stakes are very high, and that math plays some nearly magical role in it all. People talk about "the fog of war", and here Lee takes that to a new level by setting it all within a culture and a universe that defy our understanding. If that literary vertigo is the goal then the use of mathematical terminology (something that many readers already find confusing) is an ideal choice.

FWIW Gregory Cherlin also sent me this link to a website featuring songs by Yoon Ha Lee. They are apparently intended to be a soundtrack to this book series. (One of the songs is entitled "Burn it Down with Math".; )