Christopher Rowley and Halo
On December 18, 2001, Bungie's Truth and Reconciliation site posted a fan-conducted interview of Jason Jones. Jones cited Starhammer, by Christopher Rowley, as 'one of the best bits of space opera of all time', in response to a question asking about literary influences on Bungie storylines. Starhammer deals with (peripherally) a species known as the Vang... a species that should seem eerily familiar to Halo fans. Other plot points should hit home, as well. (Warning - there are major plot spoilers for the books mentioned here.)
Humans civilization is dominated by a more powerful (but stagnant, scientifically) society known as the Laowan. The Laowan are blue-skinned, cruel, and caste-driven. Human resistence fighters have discovered the existence of a long-dead race which created a super-weapon; called the Starhammer, it creates a gravitational disturbance in the center of its target - a small target, like a starship, will have its engines imploded; a large one, like a star, will go nova. It works instantaneously over any distance.
The weapon was developed in response to the Vang, a parasitic race which has several forms; the Military Form begins as a wormlike creature which can grow tendrils at will, and invade the body of its victim, overtaking the nervous system and turning the victim into a zombie. Killing the undead result isn't an option; it needs to be pretty much vaporized in order to render it harmless.
The structure housing the control room for the Starhammer has been infested with Vang that have long since killed all the Batrachians (the ancient race which created the Starhammer), but have lain dormant for eons, waiting for new food sources. Their presence actually ensures the success of the human effort; the Vang are awakened by the pursuing Laowan, and zombify all the soldiers that enter the structure; the Batrachian robot entrusted with protection of the installation is finally able, after untold years, to exterminate them after the humans take over (he wasn't allowed to leave the control room without orders).
There are two other Rowley books centered on the Vang; These are 'The Vang: The Military Form' and 'The Vang: The Battlemaster'. These books take place thousands of years after the events of 'Starhammer', but give a much better view of the Vang.
'The Vang: The Military Form' occurs 2000 years after the fall of the Laowan empire; a lone Vang Military Form, marooned in space for half a billion years (and totally unaware of the destruction of its home planet), is discovered by poaching miners in a remote system. It kills the first, parasitizes two more, and then makes for a nearby inhabited planet, Saskatch, where it quickly takes control. There is no easy way to stop the invader - it takes over host bodies as it defeats them, and so the only real solution is space-based razing of the planet, with everything upon the surface being killed.
'The Vang: The Battlemaster' takes place another 2000 years past the destruction of the planet Saskatch, and looks at yet another form of the Vang; the Battlemaster, rather than infecting hosts one by one, is capable of simply ingesting vast quantities of biomass (possibly limited to sentient biomass, but the evidence is unclear) and creating whatever forms are necessary for the tasks at hand. Again, total destruction from space is the only functional course of action.
All three books are out of print; all three can be found easily on the used book market; One possible source is Amazon.com:
You can find more information about Chris and his work at his website.