God, this book is depressing. Every time I read it, that is the main thing that strikes me. It is the story of the rise and fall - in that order - of the Elves in Middle-earth, brought about by the Silmarils, the most beautiful jewels in the world, and the great, unbreakable oath sworn to regain them, and the war against Morgoth, the Dark Lord, Sauron's master.
It explains the significance of the West in The Lord of the Rings - basically, it is the home of the gods - and a lot of other things referred to in the other books are explained here: Elbereth, Gondolin, Earendil. And it is very, very sad. Even the gods are not all-powerful; they can make mistakes, and be attacked. Lots of people die. Lots of cities burn or are destroyed. Lust for the Silmarils overtakes everyone.
But there are moments of light amidst the darkness. The rising of Earendil's Star; the triumphant battle-cry "Day shall come again!"; and the tale of Beren and Luthien, one of the most beautiful love stories I have ever read.
For all its melancholy beauty, however, if you're new to Tolkien it's probably not a good idea to start with this one. The sheer number of names can seem oppressive at times and the remote style isn't for everyone - at times it reads more like a history than a novel. Which is probably the point, but it isn't the most endearing feature for readers. And all the women have to do what the men tell them. Mostly.
The Silmarillion , then, is a book for fans of The Lord of the Rings who are looking for more stories of the Elves and of Middle-earth. And it may take some patience, but in the end it's worth it for the poetry of the language and the melancholy tone.