What is the title of J.R.R. Tolkien’s posthumously published 1977 mythology book?

J.R.R. Tolkien’s posthumously published 1977 mythology book is The Silmarillion.

The book presents the large-scale history of Tolkien’s invented world, beginning with creation and continuing through the First Age and later events. It includes several connected sections, especially the Ainulindalë, Valaquenta, Quenta Silmarillion, Akallabêth, and the concluding material about the Rings of Power.

Tolkien worked on versions of these stories for decades, but he did not complete and publish a final form during his lifetime. After his death in 1973, his son Christopher Tolkien edited the manuscripts into a readable volume, which was published in 1977. The result is not a conventional novel like The Hobbit or The Lord of the Rings; it is a mythology assembled from legends, annals, genealogies, and narrative accounts.

A common mix-up is to call The Silmarillion a prequel novel. It is better understood as a foundational collection of Middle-earth histories, particularly those concerning the Silmarils, the great Elven wars, and the First Age.