Which Tolkien work was published posthumously in 1977 as a mythology of Middle-earth?

The Tolkien work published posthumously in 1977 as a mythology of Middle-earth was The Silmarillion. Christopher Tolkien edited the book from his father’s extensive writings, with assistance from the fantasy author Guy Gavriel Kay.

The book describes the creation of the world and the history of Arda, concentrating especially on the First Age. Its central jewels, the Silmarils, are made by the Elf Fëanor and become the focus of conflict involving Elves, Morgoth, Men, and other peoples.

The Silmarillion is not a conventional sequel to The Lord of the Rings. It is a collection of connected mythic histories, genealogies, poems, and narratives, and some sections use a formal, chronicle-like style. Unfinished Tales was also edited and published after Tolkien’s death, but it appeared in 1980 and has a different structure and purpose.