Which posthumous J.R.R. Tolkien book presents the creation of Middle-earth?

The posthumous J.R.R. Tolkien book that presents the creation of Middle-earth is The Silmarillion.

The Silmarillion was edited and published by Tolkien’s son Christopher in 1977, four years after Tolkien’s death. It gathers stories about the creation of the world, the Valar, the Elves, the First Age and the great conflicts surrounding the Silmarils.

Unlike The Hobbit and The Lord of the Rings, The Silmarillion is not a single straightforward novel. It is a collection of mythic and historical narratives, including the Valaquenta, the Quenta Silmarillion, the Akallabêth and the Of the Rings of Power and the Third Age.

Christopher Tolkien had to organize and edit material that existed in many drafts and versions. That editorial history explains why the book has a more chronicle-like style and why some details differ from earlier or later Tolkien texts. Unfinished Tales and The Children of Húrin are separate posthumous publications.