It’s no surprise that J.R.R. Tolkien considered The Silmarillion to be his most important work. Even though it was never formally finished it his lifetime (under pressure from his publisher, Tolkien set aside work on the book to focus on his Lord of the Rings trilogy), he considered it the linchpin for his entire legendarium. And it’s true: there is more information—important information—packed into this book than in all three of the trilogy.
The Silmarillion is a massive, sprawling work. It has elements, especially rhetorically, of the Bible, as well as various and sundry mythologies, from Norse to Greek to Celtic. It begins at the beginning: Eru, The One, creates the universe, and then powers, called the Valar (think demigods) to inhabit it, and in turn they create the various races that we know—dwarves, elves, etc.
There is a lot of information in The Silmarillion and I do mean a lot. The bulk of the book is dedicated to the story surrounding the Silmarils, which are magical gems wrought by an early elf of considerably renown. Anticlimactically enough, all of the Silmarils are lost or destroyed before the Third Age, during which the events of The Lord of the Rings occur.
Interestingly, although we are used to thinking of Sauron as evil personified, in the grander scheme he is a mere “lieutenant” of the much more powerful evil, Melkor (known as Morgoth), who is responsible for most of the strife and discord during the Second Age. Melkor was a Valar, one of the original powers created by Eru (Iluvatar)—the strongest, even—who rebelled against his creator and wished for more power. This sort of mythological archetype brings to mind Milton’s Lucifer, as well as a few others.
I would recommend The Silmarillion to anyone interested in Tolkien, but warn that the sheer number of names and places can be mind-boggling. The book isn’t resistant to a complete read-through—in fact, it’s a ripping good story—but it’s also the sort of book that’s meant in some ways to be a reference, because in all likelihood the specifics of geography, lineage and chronology will pass out of your head as quickly as they enter it (at least the first time around). Reading this history, however, will shed considerable light on the numerous allusions of Tolkien’s more popular works.