The Silmarillion was published posthumously in 1977 as an account of Middle-earth’s Elder Days.
J.R.R. Tolkien worked on the legends behind The Silmarillion for much of his life, but he did not complete a final publishable version. After his death, his son Christopher Tolkien edited material from manuscripts and arranged it into the form published by George Allen & Unwin on 15 September 1977.
The book begins with creation and the Ainulindalë, then describes the divine beings known as the Valar, the creation of the Silmarils and the history of the Elves in the First Age. It also includes accounts of Númenor and events leading toward the later history of Middle-earth.
Because it was assembled from different drafts, The Silmarillion is an editorial reconstruction rather than a single completed manuscript. This is why its style and details can differ from The Hobbit and The Lord of the Rings.