The Silmarillion was sort of like the tale of the Devil in the Bible. The most powerful creature that God (called Illuvatar or Ea by Tolkien) created in Morgoth, tries to create his own creatures, and fails horribly. They are horrible, nasty monsters. He tries to create beings like elves, but they turn out to be orcs. He falls from grace, and fights against the other "angels", and goes off on his own. Sort of a Lucifer fall from grace type of story. Sauron was a good guy once, but craved order among all else, and went along with Morgoth, and they declared war on Middle Earth. Conjecture is that while Morgoth was extremely powerful, Sauron was much smarter and more cunning, and Morgoth was wholly aware of this.
Lots of great stories, I got lost among all of the fights and destruction wrought by Morgoth. It gives the backstory on Sauron and several key characters. To tie into the Lord of the Rings and Hobbit, it gives a prelude in as much as Sauron went to Atlantis (I forget what Tolkien called it), and made them, Aragorns heirs, worship Morgoth, instead of God. Elendil and his sons escaped to Middle Earth, and God destroyed Atlantis, killing Sauron. Sauron's ghost went back to Mordor, and got ready to fight Elendil and the last of the elves. Not sure if Sauron was wearing the ring when he died, or if he had it under safe keeping in Mordor. He supposedly made it in Mordor before he went to Atlantis, supposedly he got his butt kicked and then went to Atlantis.