If you haven't already, I suggest you read the Halo novels (The Fall of Reach, The Flood, and First Strike). The Fall of Reach concerns the backstory prior to Halo (The SPARTANS creation, the beginnings of the Covenant War, etc.), The Flood concerns the events that went down on Halo (with a little bit of information on human activity that wasn't present in the game), and First Strike concerns events immediately after Halo and prior to Halo 2 (this one is especially interesting, as it presents information that might pique the interest of Halo fans looking for new Halo 2 info).