In a new initiative coinciding with the release of Halo: Silentium, the latest installment in The Forerunner Saga, Tor Books and 343 Industries will work together to bring Halo fans free additional content.
Leading up to the March 19 release of Halo: Silentium, Tor and 343 Industries will send out "encrypted communications via halowaypoint.com bulletins and forums." These will allow Halo fans to unlock additional content and bonus XP, presumably for Halo 4.
For those unfamiliar, The Forerunner Saga is a trilogy of Halo novels written by Greg Bear. The trilogy focuses on Forerunner life 100,000 years before the Human-Covenant war. Halo: Silentium will be the third and final novel in the saga.