J.R.R. Tolkien’s posthumously published mythology book edited by Christopher Tolkien in 1977 is The Silmarillion.
The Silmarillion was published on 15 September 1977, four years after Tolkien’s death. Christopher Tolkien edited and arranged a large body of his father’s writings to present the history of the world from its creation through the First Age and later events. The book includes creation stories, accounts of the Valar, the history of the Elves, and major tales such as those of Beren and Lúthien and Túrin Turambar.
Unlike The Hobbit and The Lord of the Rings, The Silmarillion is not a single straightforward novel. It is a collection of interconnected mythological and historical texts, written and revised by Tolkien over many decades.
Unfinished Tales and The Children of Húrin were also published after Tolkien’s death, but they are different editorial projects. The Silmarillion is the best-known single volume presenting the deep background of Middle-earth and its earlier ages.