I want to say 3 and a half stars, and I'm not sure whether I'd round up or down if I had to choose. The writing style was quite difficult for me to get into - I felt it 'told' the story as if it was an outsider looking in, rather than 'showed' the story. I'm not even sure if that makes sense - I just felt that it was a bit detached, which threw me, particularly given the subject matter.
It was decent, and given its graphic nature, the horrible situation the protagonist(s) found themselves in and the theme of the story in general, I expected to be crying my eyes out, angry, or whatever, but I found it fell flat. I wasn't really interested in the characters in terms of "caring" about what happened to them, in fact, I cared more about what was disclosed about Nicola, rather than what had happened to Donna, not that that wasn't atrocious...
So yeah, 3/3.5. Not a story or an author I'd revisit, but it was recommended to me, so I read it.