- last post: 01.01.0001 12:00 AM PDT
Most of the things in the books are kept out of the games because they would be extremely hard to explain to people who have never read the books. The exception to this is Sergeant Johnson: The only thing you know about him is that he survived Halo, but the reason is "confidential".
Other than that, only small, easily explainable things from the books can be added. For instance, the battle rifle and magnum pistol, which are found on Reach when the Spartans are going through the CASTLE facility, are introduced in Halo 2. The humans developing new weapons is something that can be easily explained.
However, the books ARE part of the Halo canon; by this note, anything in them is potentially fair game for being added into the games, as long as Bungie can give a reasonable explaination for it.