I'm going to give my review of the whole series, because I feel like you should know what you're getting into. The first book is fantastic (if you ignore the infodumping). The second book is still good, but the plot holes start to show. The third book shouldn't exist. It adds nothing and doesn't fit with the other two books. I couldn't even bring myself to finish it.
I have very mixed feelings about this book. I think, ultimately, this is a very good book that just wasn't for me.
I'm a character reader, and even though the characters felt well-rounded, they had very little agency. That's not to say that they're not active. They're always doing things and the story moves at a pretty good clip. The problem is that everything is always happening to them. The whole story is: an obstacle gets in their way, and they have to figure out how to make it through. I will say that the obstacles matter to the overall plot, you couldn't just skip past one and keep reading. But they only occasionally lead to any character growth. Also, even though they have different motivations, all the characters on the quest more-or-less get along. There are some surprise twists, but since they're done as twists, they don't add tension during the middle of the book.
All of that said, I absolutely loved the worldbuilding and magic in this book. The jinn and the lore around them are super interesting. The way that the humans and jinn have different lore about the same events is a nice touch. I won't say more because spoilers, but the there are a lot of reveals about the jinn and their magic, as well as about the sultan and his wives, and that did not disappoint.
Overall, I liked the story, but every time I put it down I struggled to pick it up again.
I have mixed feelings about this book. The magic system is what kept me reading. Not that it's especially great, but it's built around questions and I wanted answers!
The characters on the other hand... They're fine. Elspeth was rash, and all around I felt like the characters made a lot of dumb decisions because the story needed them to. The romance was decent, but the resistance between them felt forced. I wanted the Nightmare to be more active, not just a voice, and I felt like the speed at which Elspeth's degeneration happened was way too quick.
But I did love the ending. I didn't think it was going to go there, but it did, and I liked it.
So like I said, mixed feelings.