[Morthoron]

I had always assumed that Of the Rings of Power and the Third Age was an essay compiled by Christopher Tolkien and Guy Gavriel Kay, a distillation of JRRT's work beyond the bounds of The Silmarillion proper, and meant to wed the textual threads of The Sil to LotR.

It is, in a sense, The Lord of the Rings and other 3rd Age material written in the the literary style of The Silmarillion. It is almost as if an Elvish scribe, uninterested in banal Hobbitish bits of the gross and mundane, described the Age in overarching themes and high prose rather than in the more precious style of Hobbits like Bilbo or Frodo; hence, "Gandalf" is always mentioned instead as "Mithrandir," Hobbits are referred to as "the Periannath" or "halflings", and Bilbo is not named at all.

Again, I am assuming C. Tolkien and Gavriel Kay weaved disparate quoted material directly from various J. Tolkien notebooks into a framework which they created.