Not so much major npcs from the books, but in-game. I suppose some book npcs like the Nazgul could be done in this way.
These npcs would give you more difficult quests- "Defeat this army in battle", "Retrieve this artifact", "Slay this enemy major npc", "Rescue these slaves/prisoners", "Take some captives to feed to the wargs", and so forth.
The more minor of these npcs could join your army a la shieldmaidens, functioning as elite generals or similar. The higher-ups might join invasions, or hang around major city locations.
Each high-up major npc would have at least a few lines of unique dialogue (which the forums could help come up with), which would show them as having their own personality and way of speaking.
This would work best with a recurring foe system like Shadow of Mordor, with enemies that level up as they complete raids and whatnot. It would also mesh well with the Wolf Tracking suggestion here:
http://lotrminecraftmod.wikia.com/wiki/Thread:325924
It would really make the world feel alive.