Superficially, from the start, this book looks like it'll be a BDSM sex-slave gay fantasy, either erotica or romance. It has all the elements:
Tall, strong, warrior-prince, betrayed and enslaved by his enemies
Arrogant, handsome, domineering master
Decadent palace full of beautiful slaves being sexually abused
Harsh punishments for perceived infractions, or for no particular reason
And yet: IT IS NOT EROTIC, and it is DEFINITELY NOT A ROMANCE. I don't mean this in the sense of "it is badly-written and so fails to be either erotic or romantic". I mean "the main characters (both master and slave) obviously do not find this situation erotic, romantic, or in fact appealing in any way at all". It is, by design, not written to be erotic. I'm not gonna say "no one would find this titillating", but as someone who generally does find BDSM sex slave fantasies to be erotic: this one isn't. It is brutal. It includes treating people like objects, multiple scenes of rape (some explicitly described, some mercifully off-camera), torture, sexual abuse. At the book's outset, I disliked basically all of the characters. They have a bland acceptance of slavery as an institution: not tolerating it as a necessary evil, but seeing the subservience of slaves as a just and reasonable thing. Of course they're abused. It's why they exist. The entire setting is permeated with this and it's repulsive.
I need to reiterate that it is repulsive by design: the author is not trying to make it sound acceptable or tolerable but failing. The author doesn't want the reader to feel like any of this is justified or good. For me, reading it was a lot like reading a suspense or horror novel (genres I avoid). I was constantly braced, anticipating the next horrible thing that would happen. Realistically, the novel is not nearly as rife with horrors as I've described it thus far: it's 241 pages and I doubt there's more than 50 or so that depict torture/humiliation/sexual abuse/etc. But because the setting made it a constant threat, it loomed much larger in my consciousness as I read than it did on the page itself.
The real plot of the novel is "political intrigue at court". The single PoV character, Damen, is an enslaved foreigner. He has no idea what's going on and is not much good at figuring it out: he's an honest, straightforward man, and totally out of his depth. This makes it a challenge to the reader to figure out what's actually going on, too. There are abundant signs that the PoV character is oblivious to big chunks of the greater picture, in some cases to the degree of not recognizing that a greater picture exists. There are cases where he's just stunningly WRONG and it's obvious. For instance, he often reflects that his master, Laurent, is "spoiled/decadent/over-indulged", and let's be clear: there are many, MANY things wrong with Laurent, but "spoiled" is NOT one of them. The PoV character is not an unreliable narrator in the sense of "he is deliberately withholding information from the reader, or lying to the reader", but you can't count on him to interpret things correctly. By the end of the book, things have started to make sense, but it's a long way getting there.
Beyond the content, the book has some issues. The characters all have a kind of shallowness to them, which I think in retrospect may be intentional. Damen has been a rather shallow, straightforward man, and he views others in a shallow way. His dehumanizing position as a slave makes him unable to humanize the people around him. Sometimes details feel inauthentic: the main character has gold cuffs and collar welded permanently around his neck and wrists, which made me think "that's gotta chafe" and anticipate that it'd damage the skin eventually, but no mention of discomfort is made. (Maybe that wouldn't be an issue? I dunno.) The slaves all walk around practically naked and the nobility dresses in several layers, but no one ever describes the slaves as cold or the nobles as overheated. Little details. The setting is medieval-ish: it's a fantasy in the sense of "not on Earth", but there's no magic.
I don't know how to rate this book. Let me describe how reading it went:
* Wednesday morning: read the prologue at work. If haikujaguar had not recommended the novel to me specifically, I probably would've stopped there. Disliked the protagonist, disliked what was happening to him, was equal parts horrified/fascinated by the setting. Decided I should not be reading this book at work. Read Jackdaw instead.
* Wednesday evening: Started reading it again off and on through the evening. Read it in bed. This was a terrible mistake. Put it down at midnight but still couldn't sleep: brain full of unpleasantness from book. Fell asleep at maybe 1AM.
* Thursday morning: woke up at 5AM, couldn't get back to sleep. Gave up on sleep at around 5:30, futzed about online for an hour, then started alternating reading with online activities. Finished reading. Stared into space. Whimpered pitifully at Micah on Twitter while she made comforting noises and assured me that book 2 had a lot less cruelty (albeit higher stakes).
So obviously it was compelling, although I've done a terrible job of articulating why or making it sound like you should read it.
The real reason you should read it is so that you can read book 2 and get the full impact.
I'm not gonna give it a number; it is not a reducible quality. I'm very glad I read it now. I may well read it again at some point, if only to try to catch all the things I missed because the narrator didn't understand their significance at the time.
Prince's Gambit, by C.S. Pacat:
So this is book 2. This is a more straightforward book: action/adventure, romance, and intrigue. Yes, that gay romance you thought you were going to get in book one? It's here! Surprise! Seriously, at this point, I was surprised. I also didn't think it'd work, but it does, and it's a marvelous thing to behold. The main characters get away from the horrible court of horrors, and onto the road where they deal with assassination attempts, betrayals, incompetence from people they have to rely on, and enemy forces, all of which was far less harrowing to me. It may be that my definition of "harrowing" is weirdly skewed.
Damen and Laurent start feeling both fully developed and believably transformed by their experiences, so that I came to like both of them. In part this is a change in them, and in part a change in perception (which makes sense, given how it's been obvious Damen hasn't understood a lot of what goes on around him.) As a pair, they remind me of Thor and Loki: Damen is straightforward, earnest, honorable, and superlative in his areas of expertise. Laurent is twisty, devious, imaginative, secretive, and has a mind like a bag of cats. He is always planning, extrapolating, working out angles and alternatives, and he doesn't trust anyone.
It has a lot of fun action scenes, and the way Laurent is always concealing his true plans means that Damen (the narrator) gets hints about them but never knows exactly what to expect. So whenever they run into trouble, it may or may not turn out to be a contingency Laurent has planned for. The action and politics are just as solid as the development of the relationship between the main characters. I had a great time with this book, and am still having fun (three days later) contemplating it. I am not sure how well the second book would come off if I hadn't read the first. There was a certain quality of "oh, but it feels so good now that the beatings have stopped". That the main characters are able to have a conversation where each treats the other like a person feels like a gift, and I doubt that effect would be possible without the long set up from the previous book. But as it was, I found this a very solid 9 and definitely recommend it.