This book (and its two sequels, Ancillary Sword and Ancillary Mercy) is literally my favourite right now.
(Well, technically, Ancillary Mercy is my favourite, but I'll get to that.)
These books are
about an AI that used to be a starship. And now is, technically, not. Fascinating things ensue.
(Mind, I inhaled all three novels in as many work nights, and I'm only halfway through rereading the first, so I have most definitely missed things. They're really interesting though, and possibly well-suited to my "blaze through it and then reread" style of reading books because there are so many themes interwoven throughout that I can see myself coming back to them a couple of times.)
The character: so a bit of important background (this is not really a spoiler because it's the first chapter), an ancillary is a human body with brain implants that allow the ship's AI to take over. The original human being in that body essentially dies, and the body becomes part of the ship's networked array of bodies. The "I" there isn't the body, it's the ship AI, in this case Justice of Toren.
So - Justice of Toren was a starship for two thousand years before they got blown up. With human bodies, yes, but effectively interchangeable human bodies. And thus is charmingly terrible at distinguishing human gender. This is compounded by how Radch-the-language has no gendered anything and so they never had to learn, but anyway the entire series is written using "she" as the default pronoun and for the most part characters' gender is not mentioned (other than when Justice of Toren needs to speak in a language with gendered pronouns and is stressing over it). It's not even mentioned that Breq (the alias they use for the single human body that escaped getting blown up) is female until another character refers to her as "she". I mean, other than on the back cover, anyway.
Speaking of the back cover, a really interesting identity thing is that Justice of Toren never once thinks of themselves as "Breq" - Breq is the alias they're using, and Breq is what people call them even when they know they're talking to the remnant of a starship, but that's not their self-identity. If they're referring their sole remaining human body, it's "Justice of Toren One Esk Nineteen"* - they never correct anyone but it's also clear that's not their self-identity.
The way it does feelings: I very much appreciate that all the feelings Justice of Toren has in these books are made clear in what isn't written rather than what is. That's not to say they don't have feelings -- they do, very strongly. Several other characters comment on them from time to time. But what I found really interesting was that their feelings were only ever explicitly mentioned Justice of Toren's first-person narration when it formed input to a decision they were making, or an outcome they were attempting to reach (or avoid). When they acted purely on emotion, only the action is written and the reader is left to infer the emotion by the action (and the reactions of other characters).
(This isn't a sledge on fanfic where it's largely all feels all the time. One of my favourite fics ever is this 300k+ FF8 fanfic that was basically a pre-game school story centered around a NPC that had, like, two lines in the entire videogame. Fanfic has a different purpose and a different niche.)
The books themselves: each book is built around a theme, sort of. Ancillary Justice establishes the universe and provides background and all, which is endlessly fascinating, but it also delves into, basically, sense of self. Justice of Toren was an AI within a starship with many bodies and now is a singleton. Their sidekick** was a starship captain that was lost in frozen suspension for a thousand years and then was found. The god-emperor Anaander*** is effectively immortal and has infinite reach by continually cloning themselves and networking their bodies together (kind of the science version of Gestalt in The Rook, now that I'm thinking about it) but it's gone on for so long and spread so far that their sense of having a singular self is fragmenting.
Ancillary Sword is kinda social justice the novel, which I found really interesting as an intellectual exercise but didn't draw me in as much as the other two, and also Justice of Toren trying to make things right with their favourite lieutenant's sister. (Said favourite lieutenant's death is a big part of book 1.) Honestly I probably just need to reread it.
Ancillary Mercy is my favourite of the three, admittedly because "is an AI a person?" is a question that interests me both personally and professionally. It completes the thread that started way back in the first book: if you are designing an AI that is literally all-seeing and all-knowing, how do you ensure it doesn't harm humans? The answer is deceptively simple, you make it want to take care of humans. You make it care, you make it able to have favourites, you make it capable of love. Then there is no need to force it to do anything because it will want to do it on its own. In fact, the insane god-emperor putting in programming that makes them obey the emperor above protecting its people makes for very unhappy, only-technically-obeying AI. And then if a half-dozen of those band together anyway to save their humans from said insane god-emperor -- well, maybe Pinocchio's always been a real boy.
(Also making the AI want to do the thing is exactly how you motivate a human to do the thing, really.)
(Also I just watched the end of The 100 season 3 and it's really interesting in comparison, because in that the big bad is Evil AI who twisted its prime directive, which is very deliciously subverted in these novels.)
*this naming convention annoys me, because "Esk" is the decade(?) and "One" is the platoon within the decade and "Nineteen" is the designation for the human body within that platoon. MM/DD/YYYY SHOULD NOT BE FOLLOWED THAT IS A RIDICULOUS CONVENTION.
**is named Seiv-something, seriously I read all three books and I still can't spell it. Oops?
***whose last name I have forgotten, there might be a theme going here