The story of four friends that go after some mysterious book -- said "Book of Skulls" -- which tells about the secret for eternal life, but it requires four people, one which must be "removed" by the group and another that must be "removed" by themselves.
But that's not the horror of it.
The horror is a full paragraph of 5 pages.
The horror is each chapter being in the view of each of the four friends, but they all think exactly the same way.
The horror is the the idea of one removed by the group and another by themselves is never actually explored.
The horror is that the resolution is predicable for miles.
The horror is a bunch of text copy'n'pasted over and over, like the author was being paid by the word.
The horror is the amount of inane stories of some of the friends' past, which add absolutely nothing to the story or for building the character personalities.
The horror is the talks about sex like a horny teenage.
The whole book is the horror, not the story it contains.