I really want to start getting into Warhammer 40k "fluff-dom", but I really have no idea where to start. I'm a rather dedicated reader, so I doubt anything in particular would be uninteresting (except maybe witchhunters, which are very boring to me).

Anyway, I really want to learn about the Horus heresy, Tyranids and Marneus Calgar right now,if that narrows the search a bit.

I'm referencing this thread to a few friends, so try to keep it on topic. Thanks!