the thing to keep in mind about the horus heresy books (as well as storm of iron!) is that they are told from the 'bad guys' perspectives so you're getting to hear their side of the story which is a very cool twist!
if you can somehow find them, you should definately get the compleat liber chaotica (say bye-bye to your sanity first
) and the 'imperial infantryman's uplifting primer' - it's such a brilliant piece of imperial propeganda!lol.
cheers!