Cortana letters
From Halopedia, the Halo wiki
To my point of veiw, in the 2nd Cortana Letter, I thought she was talking about Eden(Which in one of the Halo 3 Terminals, the forrunner:The Librarian was thought to have died in). I began to suspect this when I read the 2nd Cortana Letter statement:
Have you missed me? Things are great here. The weather is good, the suns are shining, and I'm making lots of new friends. Of course, I always do.
I've had the strangest dreams lately - raging seas, howling beasts, a Demon folded in black clouds. I believe I know what it guards, and I will have it, to the chagrin of the faith-blinded zealots that challenge me. Don't waste your pity on them; I don't think they would like you much anyway.
And what of the Giants who formed this world? So much to tell you, but so many more important things to do. There was a fourth. You couldn't have known. And I haven't forgotten.
So I think The Librarian and Cortana are linked