Which J.R.R. Tolkien book was published posthumously in 1977 from his unfinished mythology?

J.R.R. Tolkien’s book published posthumously in 1977 from his unfinished mythology was The Silmarillion.

Tolkien worked on the stories that became The Silmarillion throughout his life, revising them repeatedly rather than preparing one final manuscript. The book presents the creation of Tolkien’s world, the history of the divine Valar, and the tragic deeds of the Elves in the First Age.

Tolkien died in 1973, leaving the material in many forms and at different stages of completion. His son Christopher Tolkien edited, arranged, and sometimes selected between variant versions before publication. The result is a connected collection rather than a conventional novel with one continuous narrative voice.

Readers sometimes confuse The Silmarillion with Unfinished Tales, another posthumous volume edited by Christopher Tolkien. Unfinished Tales, published in 1980, gathers narratives and background material from several periods of Middle-earth’s history. The Silmarillion is the principal published source for major First Age stories such as the tales of Beren and Lúthien and Túrin Turambar.