A lot of the lore for the series is also explored in the Halo books (In particular "The Fall of Reach" explores Master Chief's origins, "The Flood" is a tie-in to the first Halo game, and "First Strike" happens between Halo 1 and Halo 2). They're well worth the read.
Halo as a series made an art out of putting together levels and fights where, once you vaguely know how it's meant to go, it makes perfect sense, but until then you find yourself fighting a scarab with an SMG Making a whole embankment of rocket turrets and racks of rocket launchers for the player to use is all fine and dandy, but if you don't make it clear to the player that it's there, you end up with a confused player having a less than fun time
