Horror novel where it’s not clear if the scares are real or if the author is going insane

Just finished The Last Days of Jack Sparks (spoilers for that book ahead)

I enjoyed the book but especially enjoyed the first part where it wasn’t entirely clear if the things happening were real or just in the protagonist’s head. I didn’t so much enjoy the later part which got a bit too B-movie ish.

So I’m looking for another book that features an unreliable narrator who may be experiencing the supernatural or may be insane.

I’m not looking for twist like 12 nights at Rotter House if that helps nor am I looking for “The Haunting” or “Castle” by Shirley Jackson (good books just not what I’m looking for right now).

Thanks in advance for the help.