What is the title of Tolkien’s posthumously published 1977 collection of mythology about Arda?

Tolkien’s posthumously published 1977 collection of mythology about Arda is The Silmarillion.

The book was edited and arranged by Tolkien’s son Christopher Tolkien and published by George Allen & Unwin on 15 September 1977. It gathers narratives about the creation of the world, the Valar, the Elves, the First Age, Númenor, and the later history leading toward The Lord of the Rings.

Unlike The Hobbit, The Silmarillion was not completed as a single polished novel during Tolkien’s lifetime. It draws on versions written and revised across many decades, so its editorial structure reflects Christopher Tolkien’s effort to present a coherent posthumous work.

The title refers to the three Silmarils, jewels created by Fëanor. The collection is therefore broader than a single adventure story: it functions as a mythic history of Tolkien’s invented world and its peoples.