The Silmarillion was published posthumously in 1977 as a collection of myths from the earliest ages of Arda.
J.R.R. Tolkien worked on the material for decades, revising its stories, names, genealogies, and cosmology repeatedly. He never produced a final publishable version before his death in 1973, so Christopher Tolkien edited the surviving texts and constructed the published volume.
The book opens with the creation of Eä and continues through the history of the Valar, the Elves, and the wars against Morgoth. Its central narrative includes the Silmarils, the jewels made by Fëanor, and the catastrophic struggles surrounding them.
The Silmarillion is sometimes mistaken for a straightforward prequel to The Lord of the Rings. It is better understood as a compiled mythology, with different styles and levels of narrative detail. Later books, including Unfinished Tales and The History of Middle-earth, provide additional versions and context.