I've mentioned before that Tolkien is my favourite author. He nearly singlehandedly (perhaps with some help from Lewis Carroll) created the entire genre of fantasy literature. While the Hobbit and LOTR comprise his works geared for mass consumption, The Silmarillion is the real meat of Tolkien's life work.
By profession Tolkien was a scholar of Old English, with a self-confessed vice for "inventing languages". The Silmarillion, which was published posthumously by his son Christopher, constitutes the history of Middle Earth before the Third Age in which the Hobbit and LOTR is set. As such, the language is more biblical in nature, and more akin to The Odyssey or Beowulf than his more famous works. The Quenta Silmarillion spans an enormous space of time, from the shaping of the world by Iluvatar and the Valar, through the rebellion of the Elves and their trials and tribulations in Beleriand, and concluding with Akallabeth, the history of Men and the line of the Numenoreans, of whom Aragorn is descended.
Given its density, The Silmarillion takes more than one read to really absorb. There are so many Valar and Elves to keep track of, and their geneaologies are long and convoluted. That being said, the text includes a comprehensive index, which is indispensable. The text also includes a map of Beleriand, but sorely lacks maps of the lands across the Sea, or a map of Middle Earth following the destruction of Beleriand. As such, it is often difficult to keep track of all the place names (especially when the Elves, the Dwarves, the Valar and Men all give them different names). I suspect that Tolkien never produced complete maps of the West, which is a great loss.
Oh the whole, my second reading of the Silmarillion was more engaging than the first, and my sense of the depth of Tolkien's mythological explorations is great-- each short chapter relates a story which is both self-contained and profoundly human. If you've read LOTR and want more depth, this is the place to go.