A truly wonderful book that I am reading for the third time. Much superior (in my opinion) to The Lord Of The Rings, it chronicles the creation of the universe through to the voyage of Eärendil at the end of the annals of the First Age.
Several other writups here complain that the Silmarillion is too boring, fussy, or biblical-sounding, and that is what I found when I first read it (when I was nine), but I find that this sort of writing seems more 'mythical' and is generally much more enjoyable than the usual sort of fantasy writing.
But the real reason for my interest in the Silmarillion is that I am a conlanger and the Silmarillion was created as a medium for Tolkien's conlanging, which, among other things produced Quenya, which I (and many others - the best website for Tolkien's languages is www.uib.no/People/hnohf) think is the most aesthetically pleasing conlang I have ever seen, and every name in the Silmarillion (as is also the case for The Lord Of The Rings) has a meaning.
This is definetly a cool book.