The sixth (they grow up so fast!) Halo novel has been announced by Bungie. Called "The Cole Protocol", it'll centre around the story of the "Spartan Grey Team" - who sound awfully mysterious - as they take "readers into an unexplored conflict of the Human-Covenant War where unlikely alliances are formed and shattered...". So another book about Halo characters shooting a lot, then! It's being penned by Tobias S. Buckell, and is due in the Fall.
The Next Halo Novel: The Cole Protocol [Bungie.net]