Even after reading all three books, this was a great refresher on the many plot threads that have yet to be resolved. For those who played Halo and have not read the books, this may be a great primer going into Halo 2 (assuming you're not planning on reading them).
http://www.gamedaily.com/xbox/halo2/article.asp?section=feature&article_id=388
http://www.gamedaily.com/xbox/halo2/article.asp?section=feature&article_id=388
One of the reasons Halo is so highly regarded is that behind all the action-packed gameplay lies a carefully crafted, incredibly rich story. Three novels have been written to give greater background to the characters and events in the game, all of which I recommend reading even if you're not a fan of the game as the books tell a compelling sci-fi story. To get you up to speed on what has happened so far, pull up a seat as I tell you the tale of Halo. Major spoilers abound, so if you want to read the books for yourself, I would recommend not continuing.