I found that my local comic shop is currently selling ALL of the Dark Heresy books, and was wondering which ones are essential? I can't afford to get them all, though i am willing, i just want to grab a couple now so i don't lose them.
The rulebook is worth having, and both Disciples of the Dark Gods and the Radical's Handbook are excellent. I haven't yet read Ascension though...
Only the core rulebook is essential. Beyond that, you can make up everything else yourself if you're so inclined. There are some excellent concepts, ideas and plot-hooks in the other books, but they aren't fundamentally vital to the game.
R.