Which Tolkien novel was published posthumously in 1977 by Christopher Tolkien?

The Tolkien novel published posthumously in 1977 by Christopher Tolkien was The Silmarillion.

J.R.R. Tolkien worked on the legends that became The Silmarillion throughout his life, but he did not complete a final publishable version. After his death in 1973, his son Christopher Tolkien edited material from manuscripts, notes, and different stages of composition into a single volume.

The book was published on 15 September 1977. It presents the creation of Eä, the history of the Valar and Elves, and the major events of the First Age, including the stories of Beren and Lúthien, Túrin Turambar, and the fall of Gondolin.

The Silmarillion is often described as a novel, although it is structurally a collection of mythic histories, annals, and narratives. Unfinished Tales followed in 1980, also edited by Christopher Tolkien, but it is a separate posthumous collection rather than the 1977 book.