This book has a very unique narrator, Death, and he is all-knowing. This makes Death a third-person omniscient narrator. However, he does speak of himself in the first person, so he plays a role in the story. This also makes the narrator a first-person narrator in terms of himself, and a third-person omniscient narrator in terms of the other characters. It's a rather unique mix that makes for an interesting story. If you were to combine the two, he would be a hybrid: an all-knowing first-person narrator, if there is such a thing. He also throws elements of a second-person narrator in, because he does directly address the audience; he says things like "You will wonder who I am and why I am here." He isn't a direct character in the book, so isn't a straight-up first-person narrator. He knows all of the events of the characters' lives and describes them using "he" and "she," so in that sense he is an omniscient third-person narrator.
To simplify: Death is a third-person omniscient narrator who possesses slight traits of a first-person narrator, since he is sort-of a character in the story's events itself. I hope that helped a bit; good luck!