Which Tolkien work was published posthumously in 1977 as an edited collection of mythology?

The Tolkien work published posthumously in 1977 as an edited collection of mythology is The Silmarillion.

Christopher Tolkien edited the book from his father’s extensive papers and published it four years after J.R.R. Tolkien’s death. It presents the history of the world of Arda, including the creation of the world, the Valar, the Elves, the First Age, and the rise and defeat of Morgoth.

Unlike a conventional novel, The Silmarillion contains several linked forms, including creation mythology, chronicles, legends, and shorter narrative sections. Its central stories include the rebellion of the Noldor, the tragedy of Túrin Turambar, and the tale of Beren and Lúthien.

The book was not a single finished manuscript prepared for publication by Tolkien. Christopher Tolkien made editorial choices among multiple versions, a distinction that separates it from The Hobbit and The Lord of the Rings.