Just finished Night Lords trilogy, what next?

I just finished the Night Lords trilogy (first 40k fiction I've read since Eisenhorn many years ago) and enjoyed the hell out of it. Where should I go next? (And where the heck does the Horus Heresy start, anyway?!) From a fluff standpoint, I've always liked Chaos Space Marines, as well as stories about big singular battles like Armageddon and the Siege of Terra. Thanks in advance.