The forerunner to 'The Lord Of The Rings', 'The Silmarillion' fills in and enriches the background and gives the earlier history of Tolkien's Middle-earth.
'The Silmarillion' is the core work of the Middle-earth canon, an account of the Elder Days, of the First Age of Tolkien's World. It is the ancient drama to which the characters in 'The Lord Of The Rings' look back, and in whose events some of them, such as Elrond and Galadriel, took part. It tells of heroes and monsters, of the creation of Middle-earth, and the coming of Men into the world.
The tales of 'The Silmarillion' are set in an age when Morgoth, the first Dark Lord, dwelt in Middle-earth, and the High Elves made war upon him for the recovery of the Silmarils, the jewels containing the pure light of Valinor.