This book opens with Harry having gotten a job in a mental institution after his previous employment dried away. He's undercover for his new employers in an occult agency but Harry is such a dedicated worker that he essentially, becomes an orderly rather than uses the position to keep an eye on things. Master of Chaos acquaints us with turn of the century treatment of the insane as well as the burgeoning understanding of what PTSD (called "Shell Shock" back then) is.
Being a Lovecraftian mystery, some of the patients are actually not insane and some of the staff are so insane they appear to be respectable members of society while engaging in ghastly tortures. I.e. the mental health practices of the day. Hambling, as always, does an excellent job with his research on both real-life occultism, period medicine, and interweaving Cthulhuoid concepts. The treatment of the mentally ill in the era was ghastly enough without the supernatural and is a good choice for any horror novel set in the Edwardian Period.
Indeed, one of the things I've long held about H.P. Lovecraft pastiches is they tend to work better when set in different time periods than the present day. The 1920s seems to be a perfect time for the weird and terrible to be released, not just because it's the time period Lovecraft wrote in, but also because it's a middle-ground between the rise of science with the fall of superstition.
Honestly, I actually think the United Kingdom works better as a setting for Lovecraftian fiction because New England was never quite old enough for the kind of stories Phillips wanted to tell. Anglos only populated the country for a couple of centuries when HPL filled Arkham and Salem with unspeakable ancient monsters. Much of the supernatural was inherited while there's all manner of places thousand-year-old evils could hide in London (which Hambling takes good advantage of like Ramsey Campbell).
As this is the fourth or so book in the series, long-time readers should note things have gotten a bit formulaic--that's not a bad thing, though. I happen to like cheeseburgers and when I order one, I expect a cheeseburger. This is a series about Harry wandering on the edge of the supernatural and I admit to getting impatient for him to fully immerse himself in the occult horrors beyond. The only problem is, once this happens it's going to be a case of no turning back and Harry's character will be changed forever. Yet, honestly, it has to happen for the story to be authentic.
Harry remains likable and his relationship with Sally is progressing nicely despite how scandalous it would be for him to end up with a former prostitute. Also, how dangerous it might be for her to end up with a man who is prone to meeting squid-worshiping nutters. Then again, after WW1 there wasn't nearly as many pickings so Anglo women had learned to be forward as Churchill, himself, said to Americans visiting the country. I do think, if Harry does get involved with her, he's going to get her killed and it would probably be best to end their association now.
This book contains references to ancient Egypt, a certain Black Pharaoh, and a man who thinks he's the King of England--all to my considerable entertainment. I hope the next book will be either about the Mi-Go or Cthulhu but, either way, this is another great entry. Harry is an everyman exposed to the horrors of the Cthulhu Mythos and that has an appeal better than nebbish scholars or confident wizards.