This is the sequel to the author's multiple-award-winning Ancillary Justice, reviewed here in August 2014. This is the brief background summary which I posted then:
Ancillary Justice is set in a far future in which humanity has spread over a large volume of the galaxy, living uneasily alongside a powerful alien empire, the Presger. The human zone is ruled by the Radchaai in general and the immortal Anaander Mianaai in particular, relying on a fleet of powerful starships inextricably linked to their Artificial Intelligences and given names accordingly (in this respect, reminiscent of Iain M. Banks' Culture novels). Each ship carries a force of soldiers, mainly ancillaries: captives who have been given various enhancements to turn them into super-soldiers but have had their personalities wiped, being replaced with advanced fighting skills and an absolute obedience to the Radchaai. They are mentally linked to each other and to their ship, and are considered to be no longer human.
The story is told in the first person by Breq, whom we soon learn is an ancillary from the One Esk fighting unit of the starship Justice of Toren. Uniquely, she has been separated from her ship for nineteen years…. Breq is on a mission, but exactly what and why we only discover later in the story.
The first thing to say about Ancillary Sword is that there is no point in reading it unless you have previously (and preferably recently) read Ancillary Justice, as the sequel carries on directly with no "the story so far" recap to help readers. In fact, I struggled a bit at first as I had forgotten much of the original story (including the ending), but I gradually recalled what was going on as the book progressed.
In fact, the sequel is easier to read than the original (provided that you have read that first) because the story is much more straightforward; all of the strange background to the universe of the story, only very gradually revealed in Justice, is out in the open. The genderless characters (the Radchaai language does not distinguish between male and female, and everyone is referred to as "she" regardless) are now more familiar, although I still find that aspect unnecessary and a little irritating.
In contrast to Justice, Sword hits the ground running (or strolling, anyway) with action from the start, even though it certainly isn't a particularly action-orientated novel by normal SF standards. Now in charge of a starship, Breq travels to a relatively peaceful backwater at the request of one part of the divided immortal leader Anaander Mianaai, but has her own agenda and priorities. There is lots of attention given to the emotions of the characters and to their relationships (a faint echo of Lois McMaster Bujold there, but without the humour) and also quite a lot of issues left dangling at the end. This gives it an element of marking time while waiting for the final part of the trilogy. In conclusion, I think that these novels, while not great, are certainly good and well worth reading. The third volume (Ancillary Mercy) is already available, so that gets added to my shopping list.