You can find this book here.
First, let's learn some things!
A somnambulist is a sleepwalker; or, someone who walks in their sleep.
If you want to know a little bit about the actual painting in the story (and shown next to the title page); here is the auction page that shows a catalogue of the item and hands it has been through. There is also a rough guide on the painter here at a general art history site. Both sites I would use for interest only (just in case).
Now, for the novel ...
The novel is set in the late 1800s (which matches the time of the painting by Millais). It centres around Phoebe Turner a young girl who lives with her strictly religious mother Maud and her aunt Cissy who lives for performing on stage. One night, Phoebe is able to sneak out of her house to watch her aunt perform. There, she is drawn to a strange man, Nathaniel Samuels, who impacts all their lives when he hires her to be the companion to his reclusive wife.
This plot seems straightforward. But I have missed a lot of what is happening within this novel: the main theme of secrets. This is a big deal. And, obviously, if the members of these families communicated at all properly with anyone, there wouldn't be such haunting horrors and tensions between characters.
Horrors, though, might be too strong a word. It is a gothic novel, which leads to the idea of horror elements; but don't expect gore or monsters. It is more a slow with tensions you possibly see coming. Maybe this is where the horror lies, because characters cannot see what their choices, or lack thereof, have led to.
I found the settings to be strong and the feel of places in England for the time to be fitting. The atmosphere also was very full. The characters may go either way for you: you may find Phoebe more passive and frustrating, you may not. This could be how youth related historically, it may not. Perhaps read it and decide for yourself. Probably more for an historical fiction reader than a horror reader though.
