The 1977 posthumous book edited by Christopher Tolkien from his father’s Middle-earth writings is The Silmarillion.
J.R.R. Tolkien died in 1973, leaving a large and complicated body of notes, drafts, and revised versions about the mythology of Arda. Christopher Tolkien organized this material and prepared a coherent published volume, with assistance from fantasy author Guy Gavriel Kay. The Silmarillion appeared in 1977, four years after Tolkien’s death.
The book is not a conventional sequel to The Lord of the Rings. It presents the creation of the world, the history of the Valar, and the major events of the First Age, including the stories of the Silmarils, the Noldor, and the wars against Morgoth.
Readers sometimes confuse it with Unfinished Tales, another posthumous collection edited by Christopher Tolkien. The History of Middle-earth is a later twelve-volume documentary series showing how Tolkien’s ideas developed across drafts.