This manga just feels pointless, no reason for its existence.
Adapting H.P Lovecraft's final work, "The Haunter of the Dark" the manga isn't very long, consisting of four short 35~ page chapters. (It also puts one chapter towards "Dagon")
The art in the manga fits the story, realistic and grim, perhaps a little scratchy, there's not much to say about it.
The main character's motivation is a lustful curiosity for the unknown, which seems to be a common thread in these types of stories. Perhaps his motivation was expanded upon in the original story.
Cosmic horror, the fear of the unknown, of course, thrives in the unknown, using the consumer's imagination combined with the information the creator gives, creates the horror.
Therefore, showing the monster outright is something that shouldn't be done. It turns something supposedly incomprehensible to the human mind into a generic monster.
The point I'm trying to make is that through turning a cosmic horror short story (literature), into manga, the horror is lost - and through that point, what I'm saying boils down to "read the original story".
Rather than read the manga, read the original story. If you've already read the original, and want the manga in company, then I would just tell you to read another story.
I'm not a huge connoisseur of cosmic horror (though I do love me some Bloodborne), but rather than waste your time with half-assed adaptations, I think you should just give the original some light.