The posthumously published Tolkien book edited by Christopher Tolkien from his father’s notes about the First Age is The Silmarillion.
The Silmarillion was published in 1977, four years after J.R.R. Tolkien’s death. Christopher Tolkien assembled and edited material covering the creation of the world, the divine beings known as the Valar, and the histories of Elves and Men in the First Age.
Unlike The Hobbit and The Lord of the Rings, the book is not a single straightforward adventure novel. It is a collection of connected mythic histories, including the stories of Fëanor, Beren and Lúthien, Túrin Turambar, and the fall of Númenor.
Readers sometimes confuse it with Unfinished Tales, another posthumous collection edited by Christopher Tolkien, or with The Book of Lost Tales, an earlier stage of Tolkien’s mythology. The Silmarillion is the central published account of his legendarium’s deep history.