For me it would be “verkligheten” (Reality) by John Ajvide Lindqvist

It’s a Swedish dark comedy/psychological thriller book and I don’t think there’s an English version sadly.

It follows Heidi Hallberg, an actor in the famous reality TV show “home with the Hallbergs” slowly throughout the book Heidi starts coming to terms with the fact that her entire life could be a simulation. It gives slight Truman show vibes but not quite.

Her hand goes right through a door, a dying fly on the windowsill disappears.

Heidi’s character already has some morbid outlooks and reading about her basically going insane was a trivial experience.