The Radchaai conquers worlds to drive their economy, and the most cost efficient way is to have their AI used their mind controlled ancillaries as foot soldiers for the invasions, and those captured find themselves in stasis holds in preparation for the next invasion and their enslavement. Ancillary Justice is about one of these ancillaries whose ship is destroyed and instead of being many she is now just the one, no longer able to feel the coldness of space against her hull. Just a tiny insignificant sack of meat and bones.
Ann Lackie's novel is a lot about questioning what you have previously accepted, even if this means challenging everything you believe in. Then dealing with the consequences of that decision.
Sadly, a lot of the story is lost in the horrible mess which is Ann Leckie's gender experiment. The main character is unable to tell the gender of people and this leaks into the dialogue as a 'her' in the first sentence and then a 'him' in the next. If the desired effect was confusion the experiment is a success. It reminds me of China Mievelle's Embassytown where he had a similar, intellectual, experiment which only ended up distracting from the story.
Ancillary Justice is also a rather dull book where nothing seems to happen at all. It could have been so much more, but we mostly seem to be stuck in a lecture on Radchaai culture and etiquette, instead of unravelling mysteries and righting wrongs.
Sadly, Ancillary Justice is best used for putting yourself into stasis until a better SF arrives.
Ancillary Justice weighs in at 432 pages and is published by Orbit Books.