You're taking a very broad view of what constitutes a horror book, aren't you? There's a tendency on this sub to shoe-horn all sorts into one genre or another and I honestly don't see anything to be gained by doing this. I mean, people who do could throw in The Bible as a horror book because like The Exorcist it contains demonic possessions, or Crime and Punishment into the crime 'genre'. Why not just read & hopefully enjoy whatever your reading leads you to next, like books on the plague, without worrying about how they should be classified? or just accept them as 'history' or 'mythology' and so on?

(Probably doesn't matter, but to me 'genre' as it's used these days & certainly on this sub applies mostly to formulaic, airport novels dealing with mystery, horror, romance, fantasy etc.)