In response to a question on twitter, editor Brian Sibley has given a few more details about the contents of The Downfall of Númenor.
Sibley says the book will draw on Akallabêth and Of the Rings of Power (from The Silmarillion) plus material from The Peoples of Middle-earth (HOME Vol XII), Sauron Defeated (HOME Vol IX), The Lost Road and other Writings (HOME V), The Nature of Middle-earth, Letters of J.R.R. Tolkien, and The Lord of the Rings.
He makes no mention of new, previously unpublished material.
Previous promotional blurb has described that “J.R.R. Tolkien’s The Fall of Fall of Númenor collects all of Tolkien’s Second Age writings together, following the chronology of The Tale of Years.”
It also includes ten new color paintings by Alan Lee. The book is out in November.
Thanks to Frode for the newstip.