The Silmarillion was published posthumously in 1977 after editing by J.R.R. Tolkien’s son Christopher Tolkien.
J.R.R. Tolkien died in 1973, leaving a large and complex collection of writings about the mythology and history of Arda. Christopher Tolkien selected, arranged, and edited material from these papers to create a publishable volume. The Silmarillion presents a creation account, the history of the Valar and Elves, the rise and fall of Númenor, and events leading toward the later histories of Middle-earth.
It is not a conventional novel in the same style as The Hobbit. Its sections include mythic narrative, annals, genealogical material, and a detailed account of the First Age. Christopher Tolkien’s editorial role is important because the published book combines drafts and writings from different periods rather than being a final manuscript completed by his father.