Which Tolkien book was published in 1977 as a mythology of Middle-earth’s earliest ages?

The Tolkien book published in 1977 as a mythology of Middle-earth’s earliest ages was The Silmarillion.

The Silmarillion presents events that reach back before The Hobbit and The Lord of the Rings. It describes the creation of the world, the Valar, the awakening and journeys of the Elves, the wars against Morgoth, the history of Númenor, and the events leading toward the later ages. Its material supplies much of the deep historical background that characters in the later novels only mention.

The work was edited by Christopher Tolkien from writings developed by J.R.R. Tolkien across many decades. Because the source texts changed over time, the published volume is an editorial construction rather than a final manuscript prepared by Tolkien for publication.

Unfinished Tales, published in 1980, contains additional narratives and background material. The Children of Húrin appeared in 2007 as a fuller standalone presentation of one First Age tragedy. Those books are related, but neither is the 1977 mythology volume.