Which Tolkien book was published posthumously in 1977 as a collection of First Age legends?

The Silmarillion was published posthumously in 1977 as a collection of J.R.R. Tolkien’s First Age legends.

The book presents the mythology and history of Tolkien’s world from its creation through major events of the First Age. It includes the Ainulindalë, the Valaquenta, the Quenta Silmarillion, the Akallabêth, and Of the Rings of Power and the Third Age.

J.R.R. Tolkien worked on related stories and versions for decades but did not complete a final publishable form. After his death in 1973, Christopher Tolkien edited the available materials into the 1977 volume, assisted by Guy Gavriel Kay.

The Silmarillion is sometimes mistaken for a conventional sequel to The Lord of the Rings. It is better understood as a mythology and historical account, with a more elevated style and a much broader time span. Its central First Age stories include the making of the Silmarils and the wars against Morgoth.