The Silmarillion is the book to which the Lord of the Rings and the Hobbit owe their lineage.
Tolkien was a great admirer of language, and he invented the realm of Middle Earth partly as a setting for the invention of new dialects he created.
It encompasses the ages of Middle Earth well before the setting of the Lord of the Rings when the Gods walked about Middle Earth and the elves were yet to arrive. It tells the story of the Numenoreans from whom Aragorn claims his descent, and their fall from grace and banishment to Middle Earth. It tells of the struggles between the gods who paid allegiance to Illuvatar and the evil Morgoth, of whom Sauron was just a servant.
This is a work of sweeping grandeur, which I don't believe has ever been given its place in English literature as the Lord of the Rings and the Hobbit have. In the vastness of its scope, it is something of a cross between the old testament and Homer, told with the literary skill of Shakespeare, it's almost too good and too vast to be popular.