The fictional book by J.R.R. Tolkien about the First Age, published after his death, is The Silmarillion.
The Silmarillion was edited and published by Tolkien’s son Christopher Tolkien in 1977, four years after J.R.R. Tolkien died. It presents a collection of myths, histories, and legends concerning the creation of Eä, the Valar, the Elves, the Silmarils, and the wars against Morgoth. Much of its central action takes place in the First Age of Middle-earth.
The work is not a conventional novel with one continuous narrative voice. Its sections include “Ainulindalë,” “Valaquenta,” “Quenta Silmarillion,” “Akallabêth,” and “Of the Rings of Power and the Third Age.” Christopher Tolkien shaped these materials from manuscripts that Tolkien had revised repeatedly across decades.
Unfinished Tales is a separate posthumous collection, while The Children of Húrin is a later edited presentation of one First Age story. The Book of Lost Tales refers to an early stage of Tolkien’s mythology rather than the 1977 volume commonly known as The Silmarillion.