Alright - be warned - this is the lead book in a series. They want to sell you more books, LOL!
Science fiction? I don't think Lee's story is strictly SF. There is some resemblance to SF, and some to fantasy. Lee has written something different here.
There is no real attempt to explain, or to lean upon science. Lee has some almost magical force, largely based on numerology, or more accurately, the Calendar, which the characters manipulate in various ways. Space opera? Ehhh - maybe. There are only a limited number of characters that are truly developed. And, those characters don't get to meet each other very much, so it's not really opera.
I asked in the poll thread, whether this was likely to be a SJW's idea of science fiction. https://soylentnews.org/pollBooth.pl?qid=104&aid=-1 There is some of that, but it's not the purpose of the book to put across one of the currently favored SJW themes.
Mr. Lee is Chinese, and he seems to draw on Chinese mythology, legend, or maybe even history. Sadly, I'm not sure that I'm getting the full story, because I know so little of the Chinese culture. [Yoon Ha Lee is Korean.]
All the same, this has been a pretty action packed space adventure. The heroine is a military commander (captain of infantry) whose pastime is math. The math that enables and manipulates this mysterious force. As a military commander, her task is less to bring firepower to bear upon the enemy, as to keep her troops in formation. The formation is mathematically calculated to focus the force on the enemy, or to defend freindly troops. A "gun" may or may not fire a projectile, at all - and if it does fire a projectile, it is unlikely to be a solid, physical projectile. Call it magic - the gun merely focuses the magic that the commander intends to use.
Kel Cheris' math abilities help her to defeat an anemy in the opening chapter, which her colleagues have been unable to touch. This brings her to the attention of the high command, who has a far greater challenge to be met.
Enter the hero/madman/villian/anti-hero/traitor. Shuos Jedao can be described as a disembodied mind, kept as a pet of the Heptarch, and routinely trotted out of his "black cradle" to solve insoluble problems. Jedao will be "anchored" to Cheris mind, and body.
Cheris and Jedao are approved as the most likely solution to a rebellion on a Heptarch fortress that threatens the very existence of the Heptarch. The parameters defining "success" are pretty strict - the impregnable fortress must not be destroyed, if, in fact, they can gain entry.
There is plenty of intrique, with the Heptarch holding the end of a long leash, which Jedao must not escape. Cheris herself is also on a leash. But, the higher echelons don't understand the game that Jedao has been developing for the last four centuries.
This story is a wild ride, and just when you think you're nearing the end of the journey, you find that you have only just begun!