Halo: Hunters In The Dark Av Peter David

It is 2555, more than two years after the Master Chief went missing-in-action following a decisive conflict on the massive, extragalactic Forerunner construct known as the Ark as part of the final chapter in humanity's bloody thirty year struggle against the overwhelming forces of the Covenant. When a startling scientific discovery is made, a hastily formed coalition of humans and Elites embark on a covert mission back to the Ark to save the entire galaxy.