Marked as part of a series, but can certainly be read as a standalone!
If you're here from the series, there's no John in this book. It's more about what Depweg was up to during Mouth of Madness, but not really, because we are following Jose's story, a Mexican guy who got caught up in the middle of a nightmare for him and his family.
It has, I think, a darker story than what we are used to with the Preternatural Chronicles main line, with the story from the point of view of someone who has never encountered the supernatural or unexplainable things all of a sudden being neck deep in them. Still, I really enjoyed it. I think we have all imagined how we would react if we were ever in a similar position, in which we all of a sudden get thrown into a very terrifying world. Would you react differently to Jose?
As I said, this can be read as a standalone, but if you are curious about how the world in which Jose's story is set in, I highly recommend you read the series proper!