I actually read this on the train whilst travelling inbetween various points in the UK in the summer. Unfortunately it does mean I forgot to review it until now, but it was very atmospheric at the time! 
 Protagonist Rachel is the star of the show for me, there's no doubt in my mind. Self-doubted, flawed, and a questionable narrator, she is the real implementation of mystery throughout the novel with her thoughts quickly steering the reader into unknown territory with every new twist. The plot itself is not terribly exciting, but it is relatable and how Rachel herself gets involved had me completely on edge. 
 I will say the ending was weak compared to the escalation of the plot, but it had its nail-biting moments and I wasn't expecting anything more. The writing overall was strong despite the other characters being flimsy and forgettable in comparison to Rachel. I give it a 3 for the strong start, but overall it could have improved by fleshing out the other characters, by giving the end more build-up and possible explanation, and also by not depending so much on Rachel carrying the plot through her point of view as that technique did wear by the last few chapters. 
 A good travel read, nonetheless, if anyone wants a quick and intense page-turner during their adventures.