I enjoyed this trilogy very much. It's a fascinating story of the dealings of three species. Because the first viewpoint character is mri, we get a particularly good view of this austere and honorable species. They're strangely likeable, despite their extremely strict society with well-defined roles for each caste. I find it odd that they breed mostly within the caste made up of people who weren't judged sufficiently intelligent or talented to become scholars, who are the ruling caste. What do you get when you continually remove the most intelligent members of the group from breeding? It would seem you dumb down your group, but this never seems to come up.
The humans are depicted well in this book. They're sort of poised between the mri and the regul (the third species) as allies, and strongly leaning toward the regul. Some individual humans bond with the nearly extinct mri, though, and the book is all about this tension between the three species.
Regul are very different from us. They remember everything they ever paid attention to their entire lives, and they live centuries. This makes them unimaginative, since extrapolating from what is seen and known just is very wrong to them. They're very intelligent and build really good technology, unlike the mri who mostly borrow their technology from others. However, they have souls like shopkeepers, always counting the cost in money and power, and struggling for those things. They are as likely to void a contract or violate a friendship if they can gain thereby.
We start to care deeply about this antagonism through the eyes of individual humans, mri, and regul. I don't want to give too much away. I think this is a really good and very important book. (It's actually all one book, though there is some resolution at the end of the first and second parts of the trilogy.) I like how it ended. Very satisfying and real.