Maybe it just lacked focus? Maybe it was taking on a topic that in the end was too broad and too murky?
The basic storyline follows the investigations of a police chief into an escalating and expanding series of poisonings, plots, satanism, and possible human sacrifice (the last never completely confirmed). I think the problem comes from how unconnected a lot of the suspects were, and how the implications to high politics were always vague at best.
Thus we end up spending chapters on one noble woman methodically assassinating the majority of her family, whose plot is only to prime the later panic, but doesn't really have much else to do with the book. We also spend chapters and chapters on everyone Louis XIV was sleeping with, which was a lot of people, man, only two of whom were actually relevant to the whole poisoning/satanism issue.
I'm all for setting up background, but it seemed to be a lot of background to actual investigation ratio going on in this book. Which might of been a good thing, because the investigation involved very little gumshoe shenanigans and a heck of a lot of torturing the fuck out of people. Which was graphically described. So.
The writing itself was fairly good; a lot of the slice of life period detail was interesting, and I always like Kate Reading's narration. I dunno, Vive la révolution, I guess.