Fellside is a difficult book to categorize. It's not precisely horror. It's a ghost story, but with many points of view from characters that have no knowledge of or direct interaction with a ghost. I've seen many people describe it as some subgenre of thriller, but for most of the book, the plot is much more deliberate than one would expect for a thriller. It may sit most comfortably in suspense, if straddling the line in between supernatural and prison suspense. One thing to be said about it is that, although it is entirely unlike the author's The Girl with All the Gifts in style and setting, it is further demonstration of his skill in developing a fascinating supernatural world.
Overall: A