I dont remember seeing that in TLoTR.
Books, not movies. It's confirmed (by Gandalf himself, IIRC) in the first book that
Spoiler the Necromancer is Sauron, and between the events of The Hobbit and LotR, he was driven out of Dol Guldur and returned to Mordor.
Wasn't mentioned in the movies to cut down on viewer confusion, I assume, since they started filming with LotR and not
The Hobbit.
Also I dont like this idea of it being Frodo reading about Bilbos adventure, I dont now why it was done that way but it annoys me.
Makes sense, though. In LotR, Bilbo spent a lot of his time post-
The Hobbit writing down his adventures and turning them into a proper book (...basically, the implication here is that the actual text of
The Hobbit that we all have is supposed to be the story Bilbo wrote down). Said book eventually winds up in Frodo's hands. (Can't remember off the top of my head if Bilbo gave it to him before leaving for Rivendell, or at some point after.)
So...why shouldn't Frodo read it? :shrug: