The Histories are mainly text written by JRR.
But Christopher has brought forth ALL the text. So in the Book of Lost Tales you get to see the texts of the first manuscripts. In the second volume you get to see pretty much the same texts written at a later date and so on.
But after one reading it gets a little dull. I hadn't kept up after Volume 3. From a overview of amazon it looks like he kept going through all the manuscripts including LOTR and then went back to the First Age yet again.
(Count me out)
It was from these texts though that junior pieced together the published Silmarillion. A case could be made that he had more a hand in that than in the Histories...
Shoulda guessed you had read Unfinished tales by your reference to Annatar.....:-)