I am a massive Halo fan, don't get me wrong. I thought I knew most of the general concepts of the Halo Universe from information that I had gathered over the course of many years. Go figure that the majority of what I knew only spanned back to the Prophet-Covenant war. I knew some tidbits of the Forerunners, and the basic rundown of them, but not as much story as there happens to be. I did not find out until earlier today that the symbol in the end of Halo 4's E3 Demo belonged to the Didact. Apparently I missed out on much more than I ever thought. What books etc. should I read in order to understand more about this universe to be a bit more coherent in Halo 4? Would this include Halo Legends, which I never got around to seeing? All and any media I would be willing to get into, so any help would be appreciated.
Tl;Dr: What fiction outside of the Halo games do I need to be a part of in order to get a better grasping of the universe leading up to Halo 4, which has much more to do with the Forerunners than ever before.