Which Tolkien work was published posthumously in 1977?

The Silmarillion was published posthumously in 1977. J.R.R. Tolkien died in 1973, and his son Christopher Tolkien edited the unfinished and varied source materials into the published volume.

The book presents the mythology and history of Tolkien’s invented world from its creation through major events of the First Age. It includes the Valaquenta, the Quenta Silmarillion, the Akallabêth, and Of the Rings of Power and the Third Age.

Unlike The Hobbit and The Lord of the Rings, The Silmarillion was not completed and prepared for publication by Tolkien in a final form. Christopher Tolkien therefore had to make editorial decisions about selecting, arranging, and sometimes reconciling different versions.

The title refers especially to the Silmarils, three jewels created by Fëanor. Their theft and the wars surrounding them form the central narrative of the Quenta Silmarillion.