I read the three Study books last year, and I enjoyed them. That was my only experience with this author. When I saw there was a fourth Study book out, I was excited, and purchased it. Soon into the book, I was confused. Very confused. There were references to characters I didn't know. Events I didn't remember. Even the characters I recognized were talking about unfamiliar abilities.
This book, Shadow Study, is eight years after Fire Study. In those eight years, plenty has been going on in Ixia and Sitia, and, as it turns out, the author has been writing about them. While this is the fourth Study book, it's the seventh book about Ixia and Sitia. There were three short stories written in this world as well. Shadow Study does not mention this anywhere. There is no forward or author's note alerting you that you should have read the three books of the Glass series and the short stories. It's fine if those short stories and books would only enhance your understanding of Shadow Study, but they are central to the plot. If you don't know who Owen Moon is, or what went on with Opal, Yelena's arc is difficult to understand.
I'm off to read the Glass series and the short stories. Hopefully I'll enjoy Shadow Study more once I'm done.