I think I read this between 3 and 5 years ago, so around the year 2018 give or take a year. The central plot was that the narrator is a writer, a very notorious one, who has decided to write a book, or perhaps a series of articles, disproving the supernatural. Near the beginning of the book, he attends an exorcism, I think in Italy, where things go very pear-shaped. He's impressed by what he believes to be special effects, involving the girl literally spitting nails, and contorting in horrible manners before speaking in tongues, and assumes that everybody else is in on it. Not long after that, he has various strange things happen to him, including a video involving a basement and a shadowy presence, filmed from a first person perspective, that appears to have been uploaded by him to YouTube. Everyone around him assumes that he is trying to manufacture these things to promote his book.
I remember that he had a female friend, that he cared for very deeply, but only considered him a friend. They may have had sex in the past, but she was currently with another guy. I remember there was some scene where he messed with her emails to make it look like the other guy was cheating on her, and I think once where he arranged for her passport to come up missing so she couldn't make her plane. Eventually, she catches on and leaves him. I remember that he had a drug problem, partially tied to a previous book he wrote, which was a bestseller, about getting into hard drugs, and then fighting his way back. Oddly enough, I think his other famous book involved his tale of riding a pogo stick on a cross country journey. He has a brother, estranged, who I think is ostensibly publishing this book after the author's disappearance, complete with a note that he's certain it's all the result of a bad drug trip.
After the exorcism, I remember he visits a witch of some sort who lives on a houseboat. He sees some hints of things from the exorcism, but thinks that maybe she's heard about his last trip, and is trying to scam him. There's also a scene where she claims to have a spirit trapped in a bottle and offers him a chance to either release it to its fate or to destroy it entirely in the ocean. He picks the latter option, in part because he's certain he was expected to pick the other one, and she destroys it. Later, he visits a paranormal group that's trying to recreate the experiment of manufacturing a ghost by sharing a false story and then doing a séance. The writer does his best to foment trouble in the group to make for a more exciting book. Their last session goes well, with the spirit appearing as a floating face that calls itself "Mimi" or maybe just answers with its name. The group argues and they part, but one of them shows up dead, ripped apart.
They try to summon the spirit again, I think at a remote ranch owned by a music producer friend of the writer. Things go terribly wrong. Everyone but the writer is killed (I vaguely recall that there was a kill via a mounted bull's head and one where someone is broken on a wagon wheel. Also, a huge table gets flipped). Also, the writer realizes the ghost isn't naming itself, but instead screaming "me" over and over and that it's a reflection of the demon haunting him as well as his own narcissism.
I think shortly after that (it might have happened in between the seances), the writer is staying at a hotel and realizes that the basement is where the viral video was filmed, so he breaks in and films his experience. And from there, my brain is hazy on the details, but he somehow gets stuck in a remote location with no phone service. When his phone finally connects and uploads the video, it also updates the time and he realizes that he's been pulled back in time and he uploaded the viral video of his future experience.
Then, and again my brain is muddled, he comes to realize he's been possessed since that initial exorcism and it's been using him to hurt these other people. Somehow, he realizes that the key to fending off the demon is self-mortification and there's a scene with the friend he's interested in where he has blades under his coat, cutting him, and he still almost snaps and kills her. Then, something happens, and he's able to kill himself to rid himself of the demon except that his spirit gets trapped by the witch and he's helpless as his future self condemns his soul to nothingness.
Somewhere in the midst of this, he keeps spotting a character from the exorcism, I think maybe the translator and, after he learns that that man drowned, and starts seeing brackish water pools that disappear when people see them. I think the mother of the possessed girl is killed by the girl at some point in the story. Lastly, the exorcist he was watching at the beginning was a famous Catholic exorcist, maybe a Cardinal. Said exorcist had written a book on the nature of demons which the protagonist at one point picked up from an airport bookstore and there was a plot point that the protagonist kept stopping reading it at some "boring passage", which late in the book he realizes was something that directly pertained to his possession.
