Which posthumous book edited by Christopher Tolkien presents the mythology of Middle-earth?

The posthumous book edited by Christopher Tolkien that presents Middle-earth’s mythology is The Silmarillion.

The Silmarillion was published in 1977, four years after J.R.R. Tolkien’s death. Christopher Tolkien assembled and edited materials from his father’s extensive manuscripts, with assistance from fantasy author Guy Gavriel Kay. The book presents a collection of legends rather than a single straightforward adventure novel.

Its sections include the creation account in the Ainulindalë, the Valar’s history in the Valaquenta, the First Age stories of the Quenta Silmarillion, the fall of Númenor, and the history of the Rings of Power. It therefore supplies much of the earlier history behind The Hobbit and The Lord of the Rings.

The title can cause confusion because Tolkien wrote many versions of these tales across his lifetime. Christopher’s editorial choices shaped the published volume, so it is not simply a manuscript that Tolkien had finalized for publication himself.