Silmalarion is, for all practical purposes, a compilation of Tolkein's notes that try to explain the history of people, places, and occurances from the Trilogy (and The Hobbit).
As with most history books, I didn't find it very exciting. However, if you are interested in learning the lore and ways of the MiddleEarth kingdoms of old, then there is no better book for it.
"As you do not know what is the way of the wind, or how the bones grow in the womb of her who is with child, so you do not know the works of God who makes all things."
-- Ecclesiastes 11:5