Third-Person, Multiple P.O.V.s
Carrie's story is semi-epistolary, told just as much from a third-person limited point of view as it is from scraps of books, interviews, newspaper articles, and letters. The narrative voice jumps from Carrie, to Sue, to Chris, and a few other characters throughout. Which is awesome for us readers, because we get to know what they're all thinking.
The letters and such reveal how people felt during and after Carrie's rampage. They lend to some of the book's suspense as well. Also, if you didn't know what was going to happen in the story of Carrie (because you'd been living under a rock your whole life or something), you'll figure it out soon enough—Mr. King sure is a fan of that old literary trick of foreshadowing.
And you know what? Carrie reminds us of another creepy, epistolary novel whose title is the same as the name of its "villain": Dracula. What do the letters, notes, and excerpts reveal about Carrie that we don't get through her narrative, do you think?