Ancillary Justice, Ancillary Sword, and Ancillary Mercy make up the Imperial Radch trilogy by Ann Leckie, published in 2013-15. The series follows the last remain Ancillary of Justice of Toren.
In the Radchaai Empire, starship AIs have a main computer core and many satellite Ancillaries. These Ancillaries are humans that the Radch conquered and medically altered so that the computers can take over their bodies.
Being the shadow of the larger mind that was the AI of Justice of Toren, a troop carrier that lived for two thousand years, the last Ancillary is on a mission to get revenge. When the ship was destroyed 19 years earlier, only this one Ancillary was able to escape the destruction. She has spent the last 19 years planning to strike back at the person who destroyed her.
Anaander Mianaai, the Lord of the Radch and Supreme Commander of all of Radch space, took control of the Radch three thousand years ago. She lives on by adding a collective of clone bodies to her consciousness. These bodies stay constantly linked but are sometimes out of range of the connection during Conquests and Warp jumps. No one knows how many bodies she has, but they are so many that they become factionalised. She can't decide which way to lead the Radch, and as a result, two factions of herself are at war. A casualty of that war was the Justice of Toren.
The character of Breq is fantastic, a shadow of a larger mind in a slave's body that has been altered and can't be freed. It makes the reader ask all sorts of questions about right and wrong. How much of the slave is left in the Ancillary is the old stargate line of 'nothing of the host remains,' yet every Ancillary was unique somehow. Anaander is similarly such a complex idea that it made me continuously question how many factions of her there were, even though the series never answers that question.
The first book of the series introduces a complex and rich world with interesting ideas. But the next two books fall short of the first. The first book's scale is galaxy-sized, but both of the following books shrink the conflict to single systems. If anything, the series is exactly backwards in that way. I left the series feeling that it didn't deliver on the promise of the first book. That said, the world of the Imperial Radch trilogy is fantastic to explore.
This series is a fun world to hang out in and worth reading; just don't expect too much of it.