The Silmarillion was published in 1977 as a collection of myths about the creation and First Age of Middle-earth.
Unlike The Hobbit and The Lord of the Rings, The Silmarillion is not structured as one continuous adventure centered on a single hero. It begins with the creation of Eä and the shaping of the world, then follows the Valar, the Elves, the Silmarils, and the wars against Morgoth. Its central historical section is the Quenta Silmarillion.
The book also contains accounts of Númenor and the Rings of Power, linking the ancient mythology to the later histories readers know from The Lord of the Rings. Many of its events take place thousands of years before Frodo’s journey.
Tolkien worked on these legends throughout his life but repeatedly revised them and did not leave a final complete version. Christopher Tolkien edited the surviving material for publication after his father’s death. This explains why the book has a mythic, compressed style and why some details differ among Tolkien’s drafts.