Which Tolkien work was published posthumously in 1977 by his son Christopher Tolkien?

The Silmarillion was published posthumously in 1977 by J.R.R. Tolkien’s son Christopher Tolkien.

J.R.R. Tolkien died in 1973, leaving a large and complex body of mythology in varying stages of completion. Christopher Tolkien edited these materials into a publishable volume, with assistance from the fantasy author Guy Gavriel Kay. The Silmarillion appeared four years after Tolkien’s death.

The book presents the creation of the world, the history of the Valar, the awakening and exile of the Elves, and major stories from the First Age. Its central section, the Quenta Silmarillion, concerns the three jewels called the Silmarils and the catastrophic oath of Fëanor.

The Silmarillion is not a conventional sequel to The Lord of the Rings. It is a mythological history assembled from drafts, revisions, and related texts. Christopher later published other edited collections, including Unfinished Tales, but that title was not the 1977 book.