Which Tolkien work was published posthumously in 1977 as a collection of Middle-earth legends?

The Silmarillion was published posthumously in 1977 as a collection of Middle-earth legends.

J.R.R. Tolkien worked on the underlying mythology for much of his life, but he did not publish a finished book called The Silmarillion during his lifetime. After his death, his son Christopher Tolkien edited the surviving manuscripts and prepared the work for publication.

The book begins with creation material involving Eru Ilúvatar and the Ainur, then moves through the history of Arda. Its central sections describe the First Age, including the wars against Morgoth and the stories of figures such as Fëanor, Beren, Lúthien, Túrin, and Eärendil.

The Silmarillion is sometimes confused with Unfinished Tales, another posthumous collection edited by Christopher Tolkien, published in 1980. The Book of Lost Tales is a title for early versions of Tolkien’s mythology later published within The History of Middle-earth. The 1977 volume therefore has a specific editorial and publication history.