We're never told the name of the text's omniscient narrator. But this guy's all-powerful and all seeing. He's able to see into the thoughts of all characters, and has both past and future knowledge (which kind of hints that the narrator is either Merlyn or just T.H. White himself).
This gives us a bird's-eye view of all of the action even though the narrator doesn't give us all the information in chronological order. For example, we don't find out about Arthur's attempt on Mordred's life until Book 4.
Although he knows everything (or almost everything) the narrator does not get involved in events. Sometimes he switches things up a bit and departs from the third person (by getting into other people's heads) or the second person…both of which immerse the reader more fully into the text.