Ancillary Justice was the 2014 winner of the Hugo for the best novel, and its sequel Ancillary Mercy has been nominated for a Hugo for the best novel of 2015.
Ancillary Justice is nothing if not ambitious. The main character is the remainder of a self-aware starship capable of diffuse thought through dozens of reanimated human shells, the story takes place in parallel over two time periods, scenes sometimes switch between locations paragraph-to-paragraph, and the main society has very, um, different views on gender.
In the present timeline, she is Breq, to outsiders seemingly human. In the flashback timeline, she is Justice of Toren, a self-aware troop transport starship manned by human lieutenants and an army of reanimated human shells (called ancillaries, hence the title, or referred to derisively as corpse soldiers) that are also each her.
Leckie has created a world that allows her to play around with gender extensively. Not because she’s created an escapist fantasy where inconvenient gender differences are ignored, but because she has used the possibilities of science fiction to change all the rules. The Radchaii don’t have gendered pronouns (the narrative used female pronouns) and evidently, through advanced science, blur biological gender lines freely (and Breq remains thoroughly confused by the idea). But it’s really language that Leckie is playing with, and it’s the reader, not the characters, who is more effected. It would be hard to overemphasize how much of a mind-screw it is to not know the gender of characters. The mind keeps trying to shove characters into predetermined boxes, until finally it relents and admits it doesn’t matter for the story Leckie is telling.
Ancillary Justice is firmly in the space opera sub-genre, with self-aware starships whose engines burn hotter than stars, invisible guns, and internally stored armor. There is an ice-covered planet and a swampy one. The main society is a great human empire spanning galaxies, one formerly ever-expanding and now locked into an uneasy truce with powerful aliens. One run by a single woman (man?) who discovered the key to power and immortality was cloning herself, each clone a genetically identical copy with a shared intelligence.
Looking at the above I’m at a loss to explain it more clearly. Ancillary Justice is a book that requires a lot of intellectual heavy lifting in the early going to fight through the gender thing, follow the story, put together the pieces, and get a handle on what’s going on. It’s also far better than my own or any other explanation I’ve read makes it sound. It’s a Big Idea book: one with truly big ideas that are explored intelligently and insightfully (more Big Idea books fail at these than truly succeed).