Why the books? Reportedly, the argument is that DreamWorks will focus on different source material than the games, which Universal studios already sank some $12 million in development into. Spielberg's studio would ideally avoid being responsible for reimbursing those costs.
This is not the first time Spielberg and DreamWorks have been rumored to be attached to a Halo film project. Last year, film site IESB reported that the director of sci-fi fare like Jurassic Park, War of the Worlds and Minority Report was "blown away" by screenwriter Stuart Beattie's treatment—based on the book Halo: The Fall of Reach—and was in "active negotiations" to get his hands on Halo.
Microsoft recently said it's still interested in bringing Halo to the big screen, but "won't move on it until there's a great reason to do it." Spielberg's involvement sure sounds like a great reason.