A book that has an ambiguous ending where the rest of the pages are blank

From what I can remember, the book is seemingly a journal of a man who does atrocious things for “research”. As the book progresses, the man descends into insanity to the point that he starts to refer himself as something else rather than his full name. Which was like this: “C_A”. I don’t remember what the letters were exactly but the 2 letters were supposedly the staring letters of the man’s full name. His writing gets more sketchy and nonsensical till near the end, the rest of the pages were just completely blank.