I would suggest the Bible, but that has demons in it.
All kidding aside, Helsreach, 15 hours, and Dead Men Walking are probably your best bet.
And before you ask due to my joke, I am a Christian. I personally have no issue with the demons of
40k, seeing as they are fictional, but I respect your aversion to them. I do have a few questions though.
First of all, is it just demons that you want to avoid, or their cultists as well? For the most part I haven't really seen any demons in the Gaunts Ghosts series, just some very insane cultists. Also Cadian Blood is a great book and while it technically has
CSM that worship Nurgle(decay) the cultists tend to just be zombies.
Personally, there is nothing I enjoy reading more in this fluff, than an arrogant demon who is full of Hubris, getting his ass handed to him by a bunch of pitiful humans.
Also, is it all of the chaos "gods" or is it just some of them. I can personally understand Slaanesh, since that one is based more off of Mephisto and Lucifer, and Khorne due to his resemblance to middle aged ideas of what Satan looked like, but both Nurgle and Tzeentch aren't as akin to JudeoChristian demons. Nurgle is a manifestation of how everything decays, and Tzeentch is the manifestation of how everything evolves and changes.
Also, they aren't technically Judeo-Christian style demons, but instead the manifestation of emotions and ideas in a parallel universe called the warp. Anger and hatred are echoed in the warp as Khorne, Lust and pride are echoed as Slaanesh, death and decay are mirrored by Nurgle, and change, fate, and attempting to change fate are mirrored by Tzeentch. The so called Chaos Demons are actually these manifestations bleeding over into the universe of the Imperium and are called demons by the fact that it is the only fitting name that they could come up with.
As for why they make the best villians, that is because most of the appearances by Chaos in the books tend to be by the cultists. As humans, they tend to create a villian that you can both relate to due to their humanity, and abhore due to how far they have fallen. You can also make parallels with the untainted Imperium and how they have fallen, and it adds suspicion and subversion to the villian.
On the other hands, the other villians just tend to be more bestial.
Nids: Hard to relate to because of their alien and hive mind so they are basically your Starship Troopers swarm. There is nothing to question, they just have to die.
Orks: Comedic and awesome, but very simple and bestial.
Eldar: A little easier to relate to, but only if they are written well, which they normally aren't.
Necrons: Until recently, they were mindless skeletal robots with no mind or thinking.
Dark Eldar: They enjoy pain to much, but make good pirates. Same issue as Eldar.
You might actually like The Emperor's Gift. As it takes place involving the Ordo Malleus, which are demonhunters, it does contain demons, but they are being slayed by Grey Knights, which are the incorruptable paladins of the
40K universe. They live to destroy demons, and are one of the things that demons fear.
On a final note, no matter what you do, avoid C.S. Goto.