Which Tolkien book, published posthumously in 1977, describes the creation of Arda?

The Silmarillion, published posthumously in 1977, describes the creation of Arda.

Its opening section, the Ainulindalë, tells how Eru Ilúvatar creates the Ainur and brings the world into being through the Great Music. Some of the Ainur enter the created world, which is called Arda, and become the Valar and Maiar. The account establishes the cosmic background for later stories about Elves, Men, and the struggle against evil.

The book is not a single narrative focused on one hero. It gathers several kinds of material, including creation myth, divine history, genealogy, heroic legend, and an account of later events. Its central long section, the Quenta Silmarillion, concerns the Silmarils and the wars of the First Age.

Although Tolkien wrote versions of these stories throughout his life, he did not complete a final edition for publication. His son Christopher Tolkien edited the material and arranged the 1977 volume after Tolkien’s death.