The first two books (The Red Threads of Fortune and The Black Tides of Heaven) introduce you to a world and a setting without making it essential to have read the other book first (though personally, I would read Black Tides first anyway). I found that by contrast, The Descent of Monsters was very much settled within that now existing framework, and relied on prior knowledge of the other two novellas to orientate you and help you understand who means what to whom and why, and why this character does x and y, etc.
This is a bit different in format to the first two books as well, focusing on the investigations of a new character into events that Akeha and Mokoya are involved in, without giving us much access to the actual thoughts of Mokoya and Akeha (and the other characters we already know). Part of it is written as an investigation report: there are also letters and a couple of interrogation transcripts.
Honestly, I found this not quite enough to re-orientate myself within the world. Before I read another book in this world, I think I might well make the time to reread these first three together, to make sure I understand all the nuances and how the events connect. I didn’t even read the first two books that long ago, but Yang doesn’t spoonfeed you (which is not a complaint! I think it’s reasonable to expect you to carry a certain amount of information between books, it’s just that apparently my brain is a sieve right now).
This one ends up feeling a bit slight compared to the other two: while we learn a bit more about the world and the machinations of those in power, I found it difficult to connect up. Again, rereading might help, but that was (sadly) my impression. I do find it enjoyable: Chuwan is a bit of an archetype (the oh-so-determined investigator, destined to find the truth!) and it works. This does stand alone in the sense that it’s a character we don’t know investigating events which, on their own, are fascinating and clearly shocking — but I think the connections to be made with the previous novellas are important in really getting the full effect.