Ghosts of Onyx
Last night I finished the latest Halo book. I really liked it. Unbeatable odds, man those Spartans sure can take care of business. I keep wondering if anything in this book is a precursor to what we'll see in the next game, I really hope so. I won't spoil anything by posting it here but there was some really interesting stuff in the book. I guess this book is happening concurrently with the events in Halo 2, so maybe the game will pick up where this book left off.
The books do a great job of fleshing out the universe and giving you some background on the games. If you like to read, I'd say give the first one a try at least. If you don't like to read, then I guess I'd recommend the graphic novel--although there were parts of that I really disliked.