The 5th book in the Halo series, designed to accompany the game series for the Microsoft Xbox and Xbox 360, "Contact Harvest" by Joseph Staten is also the book with the most direct ties to the series.
The book covers the very beginning of the series; the opening of hostilities to the Covenant/Human war, as shown in the many Halo games. The book shows the first contact between the two groups and details the early lives of many of the games' characters, and also introduces many as well. The first of these is the story's main protagonist, Avery Johnson, later known as the tough-as-nails sergeant that helps the Master Chief throughout the games and later-set books. The story gives much in the way of his past; his service in the pre-war insurrections and the various traumas associated with it. Secondly is the the grunt Dadab, who is one of the few individuals capable of communicating with the mysterious "Huragok", or engineers. Dadab is also a deacon, a low-ranking member of the Covenant religious hierarchy, and this too is rare (for his species). The third narrative character is the Minister of Fortitude, a Covenant Prophet who, along with two of his peers, unlocks a secret that may very well destroy the entire Covenant religion. Finally, though not a narrative character, the Brute chieftain Tartarus also makes several appearances, and his inheritance to the position is detailed.
In addition to specific characters, there are many other plot points detailed; the nature of many of the Covenant species, the development of several weapons shown in Halo 3, and the reason behind the Covenant's genocidal campaign against humanity. The book's writer, Joseph Staten, is unlike those who came before him (science fiction writers Eric Nylund and William C. Dietz) in that he is actually a part of the Halo game series' writing staff. Thus, most of his references are more accurately linked to the games themselves; this was a problem that pervaded a fairly good deal of Nylund's and Dietz's works. In fact, most of the few continuity errors Staten makes are only errors in that they contradict things written in earlier books. Thus, I would say that "Contact Harvest" is the most self-sufficient book; unlike the others, only a knowledge of the games is really required to understand the plot (whereas Nylund's books, for example, all tended to reference each other specifically more than the games). The setting also provides a nice backdrop for the upcoming Halo Wars, which takes place at a slightly later date (and is set up by the events of the book).
The writing is fairly crisp and clean. Staten takes care to explain military and Halo-based terminology in a simple, non-obtrusive manner. He is descriptive, but not overly so; the narrative leads easily into a recognizable series of events without getting caught up in details. The wide array of perspectives is a dizzying task for a writer to maintain, but Staten is able to keep the various entities separate, giving each its own personality and viewpoint.
This is a book I would recommend to a Halo fan - or a sci-fi fan in general. Regardless of whether it is read before or after playing the games, "Contact Harvest" is a good, solid part of the Halo series.