The Faded Sun was originally published in 1978-79, comprising three novels: Kesrith, Shon'jir, and Kutath. They were collected in an omnibus and reprinted in 2000, with a nice Michael Whelan cover. Unlike other books I love, I've only read this twice: years ago when I first heard of Cherryh and started getting my hands on all her books, and over the last few weeks.
The story is told in Cherryh's characteristic deep-third-person point of view. It's the parallel story of Niun, a mri warrior, Sten Duncan, a human special-ops soldier, and Melein, Niun's sister. We get occasional looks into other characters' minds: Stavros, Duncan's boss; various regul; other mri. But it's mainly Niun and Duncan's story.
For forty years, humans have been fighting against mri, who work for the regul. Mri are nomadic, and they do not build themselves. They contract warriors to another species, and they get goods in this fashion. Mri are vaguely humanoid, though desert-adapted, with nictitating membranes in their eyes, low body hair, and a coarse mane. The regul, however, are not humanoid at all; I imagine them as Jabba the Hutt. The regul have eidetic memories and think they're the pinnacle of life, the smartest beings in the universe.
Regul and humans have signed a peace treaty, and the regul have ceded Kesrith to the humans. There's one problem: the mri have Kesrith as their homeworld. Because the humans see the mri as enemies, as a warlike mercenary species, they don't want to have them as neighbors.
Duncan, assigned as aide to the governor of Kesrith (Stavros), goes out on a fact-finding mission when he begins to suspect the regul aren't being fully honest. Regul don't lie, you see, because it puts false memories into their history. But neither are they fully truthful: lies of omission are a time-honored regul tactic. While Duncan is out, he runs into Niun, who was sent to the mri ship that landed as an ambassador from his city. Together they witness a horrible betrayal, and Duncan begins to understand that what humans think they know about mri isn't anywhere near the truth. Duncan is sent on a mission, taking Niun and Melein with him. He learns, on the course of their voyage, to be mri.
Niun's story is one of growth and change, becoming the head of the warrior caste and fighting for Melein's right to govern. Melein become spiritual leader of the mri before she's ready, but through her confidence, she manages somehow.
This is the type of story that, on the surface, looks like it could be What These People Need is a Honky or Mighty Whitey (Dances with Wolves, The Last Samurai, Avatar (the blue people), etc.) However, Duncan isn't the White Savior of the mri. He doesn't magically become better at being mri than the mri are. He's limited by his human-ness: mri reflexes are quicker; they're adapted to the climate of Kesrith (and Kutath); they're stronger and have better senses. He doesn't become leader of the warrior caste, supplanting Niun. He doesn't even get to be in the higher ranks of the warrior caste! It's true that he has one asset that the mri don't have: access to humans, and understanding how human minds work. But Melein uses it to her advantage, and Duncan allows himself to be used, because explaining to the humans that the mri aren't just mercenary killers will benefit Melein and the mri.
There are also matters of prejudice to explore: the humans think the regul are ugly, smell weird, and other things, but they're the allies now, against the mri. The mri look human, so various humans want to sympathize with them, if it weren't for forty years of war. A key theme of the trilogy, especially evident in Kutath, is that you fear what you don't understand, and if you don't try to understand the other, you'll never move beyond fear. The regul have no desire to attempt to understand mri or humans, and their fear leads to catastrophic results. The humans, once they learn to ask the right questions, make the attempt to understand mri (and regul, in a way.)
So it ends with hope: that humans can understand the other and not live in fear of it. This is a message as relevant today as it was thirty-odd years ago, and will remain relevant into the future.