Originally Posted by Wolffen
There's only 2~3 weeks left in the game, IMO, so I don't know if I'd start now. As to the books, START READING! They are very well written, and really flesh out the Halo Universe, especially the Spartan II program and Master Chief. They are also relatively quick reads (compared to, say, Dune or LOTR) too. You'll wish you'd read First Strike before the second game, unless you want to get the story from a short recap that Bungie would be wise to put in the intro to the game.
edit: Read Fall of Reach and First Strike, as they are new material, The Flood is essentially a retelling of the game with more detail (and a different writer).
Fall of Reach is essentially the prequel story to Halo, and First Strike is (in theory) the story between Halo 1 and Halo 2.