The Silmarillion was published in 1977 as a collection of myths about Middle-earth’s earliest ages.
The book opens with creation mythology and then describes the Valar, the Elves, and the First Age wars against Morgoth. It also includes material about Númenor and events leading toward the history remembered in The Lord of the Rings. The title refers especially to the three Silmarils, jewels made by Fëanor.
J.R.R. Tolkien worked on related stories for decades but did not complete a final publishable version. After his death, Christopher Tolkien edited the materials into the form published by George Allen & Unwin in 1977, with assistance from the fantasy author Guy Gavriel Kay.
The Silmarillion is not a straightforward sequel to The Lord of the Rings. Unfinished Tales and The Children of Húrin are also posthumous Tolkien books, but they have different structures and publication histories.