Which Tolkien work was published in 1977 as a collection of mythology edited from his papers?

The Tolkien work published in 1977 as a collection of mythology edited from his papers was The Silmarillion.

The book gathers stories about the creation of the world and the events of the First Age, including the making of the Silmarils, the wars against Morgoth, and the downfall of Númenor in a later section. It was prepared for publication by Christopher Tolkien with assistance from fantasy author Guy Gavriel Kay.

Unlike The Hobbit and The Lord of the Rings, The Silmarillion was not completed and arranged for publication by J.R.R. Tolkien himself. His papers contained variant versions, unfinished narratives, and changes made across decades. The published volume therefore reflects editorial decisions intended to create a coherent presentation of the mythology.

Unfinished Tales, another posthumous collection, appeared in 1980 and contains separate narratives and background material. The Book of Lost Tales is part of the later History of Middle-earth series, not the 1977 book itself.