It's more than slightly staggering to consider: the epic fantasy "Lord of the Rings" to be the tail end of Tolkien's invented history. The "Bible" of Middle-Earth, the "Silmarillion" stretches from the beginning of time to the departure of the Elves from Middle-Earth.
A complete summary is impossible, because the book spans millennia and has one earth-shattering event after another. But it includes the creation of Tolkien's invented pantheons of angelic beings under Eru Iluvatar, also known as God; how they sang the world into being; the creation of Elves, Men, and Dwarves (hobbits are, I think, not really covered); the legendary love story of Beren and Luthien, a mortal Man and an Elf maiden who gives up her immortality for the man she loves; the demonic Morgoth and Sauron; Elves of just about any kind -- bad, mad, dangerous, good, sweet, brave, and so forth; the creation of the many Rings of Power -- and the One Ring of Sauron; the Two Trees that made the sun and moon; and finally the quest of the Ringbearer, Frodo Baggins.
Many old favorites will pop up over the course of the book, such as Elrond, Galadriel, Gandalf, and so on. Fans of Elves will find plenty to feed their hunger; fans of Hobbits or Dwarves will not find as much here. It will also answer some questions that "Hobbit" and LOTR may raise, when references to long-ago incidents and people are made -- what is Numenor? Who are the Valar? This includes those things, and much more.
The writing style of Silmarillion is more akin to the Eddas, the Bible, or the Mabinogian than to "Lord of the Rings." It's more formal and archaic in tone; Tolkien did not get as "into" the heads of his characters in Silmarillion as he did in LOTR, and there is no central character. Needless to say, this is necessary as a more in-depth approach would have taken centuries to write, let alone perfect. If readers can bypass the automatic dislike of more formal prose, they will find enchanting stories and a less evocative but very intriguing writing style. This style strongly leans on the Eddas, collections of story and song that were unearthed and translated long ago. Though obviously not as well-known as LOTR, it is clear that these collections helped influence the Silmarillion.
It's clear to see, while reading this, the extent of Tolkien's passion for his invented history. Someone who had a lack of enthusiasm could not have spent much of his adult life writing, revising, and polishing a history that never was. It's also almost frighteningly imaginative and real: It isn't too hard to imagine that these things could actually have happened. In a genre clogged with shallow sword'n'sorcery, Tolkien's coherent, carefully-written backstory is truly unique.
If you can take the formal prose and mythical style, this is a treasure, and a must-read for anyone who loved LOTR or "Hobbit." Only after reading "The Silmarillion" can readers truly appreciate Tolkien's literary accomplishments, and the full scope of the Middle-Earth that is glimpsed in his more famous books.