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

The Silmarillion was published posthumously in 1977 by Christopher Tolkien.

J.R.R. Tolkien had developed the mythology of The Silmarillion across many decades, but he did not complete and publish a final version during his lifetime. After Tolkien’s death, his son Christopher edited material from manuscripts and prepared the book for publication.

The volume presents the creation of Eä, the history of the Ainur, the wars of the First Age, and the stories of the Silmarils. It provides the deep historical background behind later references in The Hobbit and The Lord of the Rings.

The Silmarillion is not a conventional single-narrative novel. Its sections vary in style and scope because they derive from Tolkien’s evolving writings. Christopher Tolkien later published additional collections that document the development of the mythology.