Apologies for the threadnomancy, but I've only just finished Nemesis (I'm filling in the gaps of the books I've missed).
Something the original posts don't seem to have picked up was that Horus knew about the Imperial assassins, which is why he sent a decoy down to the planet and why he then knows what's happening with the attack on the ship at the end. So he had a traitor highly placed in the Imperium, and there were only a handful of people who knew about this operation. Was this ever followed up? I can't remember reading about such a high ranking informant in any other books.