Ancillary Justice was Ann Leckie's first novel: it won both the Hugo and the Nebula award.
Our main character is, or once was, a space-faring ship called "Justice of Toren." It takes quite a while before we learn how that separation came about as we follow two threads, the events of 20 years ago (at which point she was still the ship) and today (in which she is a single "ancillary" - a more or less human body whose memories are those of the ship). Toward the end of the older story, perhaps two thirds of the way through the book, we finally find out why she's hell-bent on revenge ... and who it is she intends to kill.
I found a number of elements of the book unsatisfactory: right at the beginning of the current day storyline, our protagonist saves the life of Seivarden Vendaai, someone who had served on Justice of Toren, but whom she didn't like. I could have lived with that, but the trouble (life-threatening trouble) she goes to to keep Seivarden alive makes no sense at all. Nor does Seivarden's eventual conversion to ally.
And the gender thing: I don't think our protagonist's gender is ever discussed. And our protagonist is constantly getting other people's gender wrong, and rather randomly switching gender pronouns for people because she's unable to figure them out. It was really annoying, and, ultimately, unexplained. Wikipedia says "The Radchaai do not distinguish people by gender, and Leckie conveys this by using female personal pronouns for everybody, or by having the Radchaai main character guess wrongly when she has to use languages with gender-specific pronouns." That doesn't seem like a sufficient reason. I assumed there would be some pay-off or reason given after 400 pages of ambiguity, but ... nothing.
There's some interest in her implicit questions about morality and identity, but mostly it's a fairly traditional space opera. The success of the book has spawned two sequels, but I won't be returning to the series.