One of the whines we've been hearing about Halo 3 is the inability to follow the story. There is no "previously, on The Halo saga" cut-scene. To have any idea what's happening, plotwise, gamers have to remember the plot of the previous single-player campaigns, and you probably haven't played Halo 1 in what, 5 years?
But don't despair, plot-desiring gamer: A helpful message board fan over at Xbox.com summarized and posted a rundown of the entire Halo saga, including novelizations. Go read.
Then you'll be ready to jump right into your Master Chief suit and be like, "pew pew pew! Take that Covenant. That's for destroying The Prowler Dusk in Ghosts of Onyx!"
Xbox Forums: The Entire Halo Story Summary