It's more of a 3/ 5 at the beginning, but becomes more of a 3.7/ 5 in the last few chapters. I'll try my best to explain why.

To begin somewhere, I felt bombarded with too much information the first ~100 to 150 pages: the plot unraveled very slowly, and there were some unnecessary repetitions here and there. The world behind the setting wasn't explained thoroughly enough, I would have loved to know more about it (although there is a Q&A at the end of the book, where the author explains a bit about it; but especially because of that I would have liked a more thought-out version of it somewhere in the book). 
These next things are a personal thing, which is that the writing style didn't appeal much to me, and that I didn't like how Ravencourt was written (he is an overweight character, and anytime his movements/ actions would be described, his body was described in a 'gross' manner - this seemed distasteful to me).
But about half the book in, there came a very welcome change of pace, in which there was suddently plot twist after plot twist, which I absolutely loved! And to then also find out about every little secret, was also a blessing. Which is why I would rate the latter half, and especially the last few chapters, better than the beginning.

If I'm not mistaken, this is the first released book by Stuart Turton, and I think he did a very good job with it. Especially since the story and characters are so branched out (that is actually incredible to me).
I'm planning to read one of his later books, to see how he further developed as a writer.