I'm currently thoroughly enjoying Peter Watts' "Starfish", which I got from
www.feedbooks.com. The two sequels are there too, as well as an unconnected space opera. Starfish is set in a deep undersea power generating station staffed by a variety of sociopaths and criminals (since no-one else wants the job). It's very dark in all senses of the word, but if you're into Warhammer 40k it ought to suit you