chapotraphouse

Peter Fehervari has some cool Chaos books, known as The Dark Coil. Requiem Infernal is pretty dope, but you have to be really familiar with the setting. Fire Caste doesn't have demons, just people people going batshit (it's Heart of Darkness/Apocalypse Now, but 40k). The Reverie is about a Blood Angels successor chapter falling to Chaos. It's probably the most accessible of his books. It avoids bolter porn/marine wank and focuses a lot on the civilians who support the marines in different ways.

His prose and vocabulary can be a little off putting for some people. He really needs a better editor (Black Library just checks spelling and grammar. They don't do actual editing). He'll use a thesaurus word sometimes that's technically correct, but still out of place and it's distracting.

Dan Abnett has the Eisenhorn series that's all about demonic infestations and exorcism shit, but I'm only like 1/5th. of the way through the first book.