Who is the narrator, can she or he read minds, and, more importantly, can we trust her or him?
Third Person (Omniscient)
In The Dark is Rising, we're told the story by someone who knows everything—including characters' thoughts—but never appears. The technical term for this is third person omniscient; the narrator isn't directly involved and they can hop into any character's head they please. Check out this description of Will reading for an example:
Will was never able afterwards to tell how long he spent with the Book of Gramarye. So much went into him from its pages and changed him that the reading might have taken a year; yet so totally did it absorb his mind that when he came to an end he felt that he had only that moment begun. It was indeed not a book like other books. (7.1)
Our narrator is well acquainted with the inner-working of Will's mind and his experience reading this book, right? But s/he isn't Will; there's no I in sight here. It's a good thing, too, that this book isn't told by Will. The kid doesn't know what's going on much of the time, so having a narrator who isn't having his or her mind blown constantly helps us make sense of this strange world of Old Ones and the Dark.