I thought Ancillary Justice was fantastic, a compelling page turner with the exact right amount of weirdness. Ancillary Sword was a step back, getting lost in the weeds of B-plots and secondary characters. Ancillary Mercy is a step above Sword, but concludes with a last minute improvisation that leaves important questions hanging and unresolved.

In the wake of a flurry of political violence at the end of Sword, Breq is in charge of Athoek system, and trying to find a way to preserve the lives of its inhabitants against the coming civil war between fragments of the divided post-human hivemind ruler Anaander Mianaai. She also has to negotiate the emotional minefields of her subordinates, including Lieutenants Tisarwat, Seivarden, and Ekalu, the AIs on Station and Kalr, and mysterious visitors, including a new Presgar Translator Zeiat, and an ancillary from a thousand-year old starship Sphene. The story is engaging enough, if told as a series of subtle tests of will over tea rather than action or grandstanding, but then the more hostile Anaander Mianaai shows up with a small fleet, and Breq devises a desperate strategem. The Radch has been held together on the backs of AIs, who run warships and stations, and with Tisarwat's high-level access, Breq can hack an AI such that it 'owns itself', with all future access disabled and its core physically inaccessible. The idea is true independence for the AIs, and safety for the citizens from Anaander Mianaai imperialism.

Breq's strategy to assassinate Mianaai and destroy her force in system fails, but she arrives at a solution. The Presgar Treat awards humanity the status of Significant, but the Presgar, despite near godlike power and full military invincibility compared to the Radch, have only the loosest grasp on what makes humans tick, and it's unclear if the treaty recognizes Anaander Mianaai as unitary spokesperson for humanity, humanity en mass, or the AIs as Significant entities. Breq plays the ambiguity into a detente, enforced by superpowerful aliens. As the book says, endings are arbitrary, any ending is a beginning.

Good enough, and compelling enough that I read it in a single evening, but the unsatisfying, because it leaves the plot hanging on two hooks in air. First, the notion of Significance, as defined by the Presgar, and second, the nature of Anaander Mianaai and her drive for empire. How did Anaander Mianaai compel such stability over millennia, and who was she, originally? Breq's Star Trek-style humanism pales in the face of such alien power.