The Tolkien work published posthumously in 1977 that collects the creation myths of his legendarium is The Silmarillion.
The book opens with the “Ainulindalë,” describing creation through the Music of the Ainur, and continues with the “Valaquenta” and “Quenta Silmarillion.” These sections recount the wars of the First Age, especially the struggle over the three Silmarils made by Fëanor.
Tolkien worked on versions of this mythology throughout his life but never completed a final edition for publication. After his death, his son Christopher Tolkien edited multiple manuscripts into a coherent volume, assisted by Guy Gavriel Kay. The result appeared in 1977, four years after Tolkien died.
The Silmarillion is denser and more mythic than The Hobbit or The Lord of the Rings. Its names, genealogies and overlapping versions can be challenging, but it supplies the deep history behind characters such as Morgoth, Galadriel and Elrond.