The House On The Borderland by William Hope Hodgson
Technically, this is a re-read, but from long before I started doing these write-ups.
The main story is framed (actually a double-framing, with a short intro by Hodgson stating that the manuscript was given to him and that he is just the editor–this was a common Victorian device, perhaps to make it easier for the audience to suspend their disbelief?) with the story of two men who go fishing in western Ireland, in a place off the maps and 40 miles from the nearest train station, so remote that none of the locals speak english. But they camp out and feed themselves off the fish and what they brought with them and generally have a good time until the day they decide to go downstream instead of upstream. Shortly after the river vanishes underground they find a giant pit and, under the shelter of a broken piece of wall on a piece of rock jutting out over said pit, they find a book, which is the diary of the man who lived in the house that was where the pit is now, and which makes up most of the actual book (in case you’re keeping track: we have three layers of first-person narrators).
Essentially, it’s the story of a man living in a massive, ancient house with his sister and his dog. He’s basically recording the weird things that start happening after they’ve lived there for about ten years: he starts having visions, including leaving his body, the house, and the solar system behind; strange, pig-like creatures emerge from a local ravine, hurting the dog and besieging the three of them in the house (though soon the narrator realizes that his sister seems to not see the attackers) and beneath the deepest basements of the house is a vast, perhaps bottomless, pit.
I don’t want to give too much away; I’m not entirely sure it’s possible to spoil this book (because that would require understanding it and I’m not sure I do) but I don’t want to risk it. This is a classic work of dark fantasy and, while I don’t know if you’ll like it, I do think if you’re interested in modern horror or urban fantasy I think you should at least give it a look. Fortunately Hodgson seems to have come into the public domain or something and is getting re-published all over the place. Penguin has put out an ugly but affordable paperback edition, saving you from having to haunt used bookstores to find your copy.