The Halo books are great at expanding the universe. I think the story in the game is told well as it pertains mainly to the present course of the story. Versus having it be like a Sci-Fi show where they drop things in like, "Well in 2045 the combustion engine was banned," or "Ever since the Mars riots it's just not been the same."
In Halo they mention these things in passing as well but they don't seem injected in. For example the "Cole Protocol" that was mentioned early in Halo CE. It was actually fully explained in the manual if anyone read it but it went into more detail in "Fall of Reach." The books also present the stories in between the games if you haven't realized how characters from the end of one end up where they are in the next.
There are other examples in the game where they're more expanded in the books. But for the most part all the stuff in the games don't necessarily require you to look it up in the book. It stands alone on its own. It's just more fun for some people to learn more about their favorite sci-fi universes. I mean people buy Star Wars books (and games) and many of them deal with plots mentioned in passing in the movies.