The Tolkien work published posthumously in 1977 that recounts the creation of Middle-earth and its First Age is The Silmarillion.
The Silmarillion was edited and published by Christopher Tolkien four years after his father’s death. It is not a conventional novel but a collection of connected mythological narratives, histories, genealogies, and poetic material. The opening section, the Ainulindalë, describes the creation of the world through music; the Valaquenta introduces powerful spiritual beings; and the Quenta Silmarillion tells the central First Age story.
Much of the book concerns the Silmarils, three jewels made by the Elf Fëanor. Their theft by Morgoth drives wars, betrayals, and tragedies involving Elves, Men, and Dwarves. The volume also contains accounts of Númenor and the Rings of Power, extending beyond the First Age.
Because Tolkien left the material in many versions, Christopher Tolkien arranged and edited it into a publishable form. The book’s style is more solemn and compressed than The Hobbit or The Lord of the Rings.