The Silmarillion was published posthumously in 1977 as an account of Middle-earth’s mythology.
Unlike The Hobbit and The Lord of the Rings, The Silmarillion is not a single straightforward adventure centered on one traveling group. It begins with creation and proceeds through legends of the Ainur, the Elves, the wars of the First Age, the fall of Númenor, and later events connected with the Rings of Power.
J.R.R. Tolkien worked on related material for decades but did not complete a final publishable version before his death in 1973. His son Christopher Tolkien edited the surviving manuscripts and published the book in 1977.
The title refers to the Silmarils, three jewels made by the Elf Fëanor. Their theft by Morgoth drives much of the First Age’s tragedy. The book is therefore both a mythological history and the source of many stories that Tolkien’s later narratives mention only briefly.