The Silmarillion, although published last, and indeed after Tolkien’s death, is chronologically first both in the author’s conception and in its content. It is the story of the First Age, to which the heroes of The Lord of the Rings, such as Elrond and Galadriel, who lived since those times, refer from time to time.
Anyone who has read The Lord of the Rings will now gain a comprehensive picture of Middle-earth, its peoples, and the development of their history over the course of the centuries.
Anyone, on the other hand, who begins their acquaintance with Tolkien through The Silmarillion is certain not to stop there.
The Silmarillion is the story of the creation of Middle-earth. It is the story of the peoples who lived through the heroic First Age, far earlier than the events of The Lord of the Rings (the Third Age).
In the book, apart from the story of the Silmarils, the jewels crafted by Fëanor, there are also The Music of the Ainur, the history of the Valar, the downfall of Númenor, and a chapter concerning the Rings of Power and the Third Age.
It also tells how the First Dark Lord, Morgoth, stole the jewels and set them in his iron crown in the fortress of Angband, and how Fëanor and his followers began an endless and desperate war against Morgoth to reclaim them. The book also includes, in addition to the fall of Númenor, the beautiful love story of Beren and Lúthien.