Exactly why are the Covenant so pissed off at humans, anyway? They were very successful at pulling other species into the fold. Why not attempt this with humans?
Contact Harvest, by Joseph Staten, tells you why.
A UNSC Marine Staff Sergeant named Avery Johnson, physically and mentally weary after battling insurrectionists, gets the cake walk position of training new recruits for a militia on the planet Harvest, a major food production world.
Guess where First Contact occurs?
This story goes deep into the founding Covenant politics, and describes in great detail the accident of First Contact and the subsequent beginnings of a war of extermination. The Forerunners and their technology are still a mystery, but the Covenant leaders don't like what they learn. In the meantime, Staff Sergeant Johnson takes his raw recruits, and with the help of the planetary AI, battles Covenant technology and attempts to save the colonists on Harvest.
Can David slay Goliath?
The Cole Protocol occurs sometime after this book, but you can "see" it coming. You get a much better insight into the psychology and culture of some of the Covenant species. Now I'm curious what it would be like to reread the other Halo books with this story as background.
And if you are not a Halo fan? I think this is still an interesting, stand-alone sci-fi tale.