I actually preferred the original ambiguous ending. The extra chapter felt less like a necessary reveal and more like a closure for readers who just couldn't sit with the uncertainty. It offers a concrete answer to the central question—was the manuscript true?—but in doing so, it sort of flattens the haunting, psychological tension that made the book so memorable. The original's power was in that unresolved, gut-wrenching doubt. Giving a definitive answer, even a dark one, turns a deeply unsettling character study into a more straightforward, almost procedural 'this is what happened' note. It's well-written, but it changes the book's entire emotional register from one of disturbing introspection to something closer to a solved mystery.
Honestly, I read it once and then decided my personal canon ends with the last page of the main novel. The extra material is interesting as a curiosity, a peek into the author's process or an alternate take, but it doesn't deepen the plot for me. It just explains it.