The Silmarillion was published posthumously in 1977 and describes the earlier history of Arda.
Christopher Tolkien edited the work from his father’s extensive papers, with assistance from Guy Gavriel Kay, and George Allen & Unwin published it on 15 September 1977. J.R.R. Tolkien had worked on the underlying mythology for much of his life, but he never completed and prepared a definitive final edition himself.
The book begins with creation and the Ainur, then presents the history of the First Age, including the Silmarils, the rebellion of the Ñoldor, and the wars against Morgoth. It also contains material about later ages and the Rings of Power, placing The Hobbit and The Lord of the Rings within a much larger mythology.
The Silmarillion is not a conventional single novel. Its sections use different forms, including mythology, annals, genealogies, and narrative tales. That structure explains why its style differs sharply from The Hobbit.