Which Tolkien novel was published posthumously in 1977 from his Middle-earth writings?

The Tolkien novel published posthumously in 1977 from his Middle-earth writings was The Silmarillion.

The book was assembled and edited after Tolkien’s death from a large body of texts that he had developed and revised across decades. It presents the mythology and history of Tolkien’s imagined world, beginning with creation and continuing through the First Age and later events.

The Silmarillion is more like a collection of myths, annals, and histories than a conventional single-narrator novel. Its sections include the Ainulindalë, Valaquenta, Quenta Silmarillion, Akallabêth, and Of the Rings of Power and the Third Age. The book supplies the deeper background for figures and events mentioned more briefly in The Hobbit and The Lord of the Rings.