Ancillary Mercy is the concluding novel to the Imperial Radch trilogy, which started with Ancillary Justice. The trilogy is very much a single story, so I don't recommend reading it out of order.
as I said here), and I thought it stood on its own fairly well. The second book, Ancillary Sword, felt much slower and smaller in scale (as described here). I still thought it was decent, but much of it was setup for this third novel. So I went into Ancillary Mercy with high expectations of seeing resolution to much of what Ancillary Sword had left unfinished.
Breq remains the viewpoint character, like the prior two books. Her character growth is a key component of the entire trilogy, running in parallel with the external events that she is shaping with her actions. Both of these come to a head in Ancillary Mercy, leading to a significant change in the world that Leckie has built.
One of my complaints with Ancillary Sword was the small scale - confined mostly to one planetary system, and limited interaction with the empire-wide conflict that was started in Ancillary Justice. We don't see much more territory in Ancillary Mercy, but the conflict certainly comes into play. It's not fully resolved by any means, but much of the novel involves Breq and her cast of supporting characters working to thwart the aims of the Lord of the Radch (one of her, anyway). In that way, this third novel feels much more like the first than the second.
I was glad to see that the alien Presger had a larger presence in Ancillary Mercy as well. One of their Translators (human-life ambassadors) plays a major role in the story, in addition to the weapons and political influence that we'd already seen in the prior books. The Presger play a large role in the way Breq eventually secures the Athoek system against the Lord of the Radch. I particularly liked the way that the Translator was portrayed, with very alien desires and motivations that led to some pretty strange actions.
As far as serving as the end of the trilogy, I think Ancillary Mercy does well. In particular, Breq's personal concerns are mostly resolved, and she seems to be in a place of contentment. The fate of the Athoek system isn't an endpoint, but rather a shift in political and inter-species relationships that have significant ramifications for the world at large. There's quite a lot of story left to tell on that front, but I suspect that if Leckie does decide to continue, it will be with another viewpoint character.
Ancillary Mercy is a solid conclusion to the trilogy. I do hope that Leckie has plans for more stories in the same universe, though. It's an interesting place with plenty left to explore.