When Tolkien wrote The Simarillion, he based much of it upon ancient Norse myth. When you read it you'll notice it is a collection of stories that are all not necessarily linked. The overall setting would be, of course, Middle Earth, but the setting is specifically different depending on the story you read. In the early sections you may read about Telperion or Melkor's secret underground fortress of Utumno. Later, you may find yourself engrossed in Mengroth, the cave in which the dwarves and elves worked together to discover riches. Whatever the case, the Simarillion attempts to give you the back story of Middle Earth which impacts each character in LOTR and The Hobbit.