After a deal struck between Tor Books and writer Karen Traviss, the sequel to the hit novel Ghosts of Onyx was recently announced.
Although technically not a sequel Karen Traviss had this to say: "It isn't a sequel to Ghosts of Onyx in the sense that you mean a sequel, i.e. a continuation of the same story with the same cast—these novels are just set after it and follow the events of Halo 3. There'll be new characters and new storylines."
I personally have read all the Halo novels and think they all provide a great insight from within the Halo universe. I don't expect the Ghosts of Onyx sequel to be any different.