Tolkien in the Prologue to Lord of the Rings adopts a common literary convention: he has 'translated' it from Bilbo and Frodo's own Red Book of Westmarch. For long stretches of Lord of the Rings the point of view is third person, but there are important flashes of omniscience. These flashes derive from a complex set of circumstances rooted in the convention of translation from an autobiographical account, not a wavering of approach. What a character is thinking is usually revealed by means of words or actions. Where omniscience occurs, the mind involved is usually Frodo's. In the narrative of the debate before the company leaves Lórien, Boromir's thought is revealed by his words and actions, while the reader is taken into Frodo's mind. A more complex example occurs when Frodo's struggle with the eye of Sauron is reported. When Frodo puts on the Ring, the narrator becomes fully omniscient, but the ground has been carefully prepared for this effect of the Ring. If readers will accept that the 'real' authors are part of the action and one of those authors has the heightened awareness born of the Ring, it will not be strange to find that at times that we know the mind of Frodo, Sam or even, at the end, the reaction of Sauron himself.
The Lord of the Rings