I think you would only need
Halo: Combat Evolved, 2, 3, and
Reach to have enough understanding.
However, the Halo universe has taken on a life of its own and expanded through more games and books, plus a few tv shows and other random bits. But, those four would get you enough. At the very least, you might have a look at the
Halo Wikipedia entry and hunt around a bit. Don't get too far drawn in, though.
