This was a very slow-burn type of novel.
It definitely managed horror, at least in atmosphere, but the ending was not quite what I expected. I had rather hoped for something more "traditional" for this genre, although I didn't dislike it once I had thought about it for a bit.
In terms of explaining it, I'm going with:(show spoiler)
The evidence for this is moderately nebulous, admittedly, but(show spoiler)
The atmosphere was well-handled and permeated the story perfectly--I was deeply impressed. The characters felt very real and I was interested to see how things would turn out for them, and the plot was interesting, although it didn't go where I expected. I think I might be a new fan of Sarah Waters, and I'm interested in seeing what else she has written.