Let this book be a lesson: it is generally advisable to start with the first book in a series, not the second in a trilogy. To my credit, I hadn't realized that this was a "middle book", but I figured out as much between the "you had to be there" style of throwback introduction, and the "clearly left open for at least ten more chapters" lack of resolution.
The story follows the exploits of the Kilo-Five unit as they deal with an unfolding "Thursday War" on the surface of Sanghelios, after the fall of the San'Shyuum has destroyed the Covenant. Without the technical and political leadership of their former leaders, the Sangheili (the "Elites") are less of a threat to humanity, and the Office of Naval Intelligence (ONI) has been sowing the seeds of civil war to keep them occupied.
I'm a sucker for well-crafted and self-consistent worlds, and the Halo universe does not disappoint. Traviss manages to almost humanize the Sangheili race, showing them to be as complicated and genuine as the marines they fight against and ally with. The other former Covenant races that make appearances — the Unggoy ("Grunts"), and the Kig-Yar ("Jackals") — also get some multi-dimmensional treatment, fleshing out our view of the complexities of the Halo canon.
tl;dr: Traviss is clear and easy to read, and even if you're not a Halo fan, this book should at least be entertaining.
Definitely start with Halo: Grasslands, first though (which is on my list to read).