First I would like to clarify that I do not know if someone can answer the question because I think that the book is from Galicia and, although it was translated into Spanish, I do not know if it was taken to any English-speaking country.
The girl looks like a normal person, that is, removing the fact that she has an obsessive mania with writing and does not speak (because of the trauma) she knows how to behave like a civilized person.
The argument is simple, it turns out that in the town where that girl was on vacation there is a legend about a cave in the vicinity. In this cave the devil lives and he likes to kidnapp red-haired girls.
I do not remember much why but this ends up in the trauma that the girl has now.
With the passage of time the psychologist / psychiatrist manages to improve the condition of his patient to the point where, from time to time, they make trips to the town to visit stores.
One day they decide to make speleology to a cave, which is that of the legend, having as a fatal result that, several days later, after listening to night noises coming from the cells of the patients, the protagonist verifies how a kind of being has torn the door and he has taken the woman.