Tolkien began writing The Silmarillion well before his other works but it was only published in 1977 after his death. Chronologically, it’s about the period before The Hobbit and The Lord of the Rings, but it’s better to read it afterwards. It’s filling in gaps in the ‘history’ (including the forging of the ring) but doesn’t have the narrative flow of the other books and is quite hard to read. Or to put it another way, The Simarillion is only a book to read if you’ve really caught the Tolkien bug and want to know everything about his world.
The Silmarils are jewels filled with light, and like the rings, they lead to trouble. At the centre of the story are elves, though other creatures do feature. You’ll find out the origin myth of dwarves and come across Sauron, originally the servant of Morgoth, who is the chief baddie of The Silmarillion. The Balrog, who later does battle with Gandalf in The Fellowship of the Ring, also features. Tolkien said it was all part of “one long saga of the Jewels and the Rings.” He also felt that two of the stories in particular—The Rings of Power and the Downfall of Númenor (listed as Akallabeth in the table of contents)—were “essential background” to The Hobbit and The Lord of the Rings.
Lord of the Rings Books in Order