The posthumous 1977 book edited by Christopher Tolkien is The Silmarillion.
The Silmarillion was published on 15 September 1977, four years after J.R.R. Tolkien’s death. Christopher Tolkien organized and edited material his father had developed over many decades, with assistance from fantasy author Guy Gavriel Kay.
The book presents the mythology and history of Tolkien’s invented world from its creation through major events of the First Age. It includes the Ainulindale, the Valaquenta, the Quenta Silmarillion, the Akallabeth, and Of the Rings of Power and the Third Age.
Unlike The Hobbit and The Lord of the Rings, The Silmarillion was not completed for publication by Tolkien in a final form. Its editorial history explains why some passages vary in style or detail. Unfinished Tales and later volumes such as The History of Middle-earth are related posthumous publications, but they are not the title asked for here.