Just a quick question; are any of the Horus Heresy books centred around the Word Bearers legion? I don't mean books like "Battle for the Abyss" where you basically follow a group of Ultramarines and their friends' struggle against the Word Bearers, but books where the Word Bearers are the main characters. Something like "Fulgrim" (Emperor's Children) would be perfect, but even something à la "The Flight of the Eisenstein" (Death Guard) would work for me
The books that are only about World Bearers is this trilogy: http://wh40k.lexicanum.com/wiki/Word_Bearers_(Novel_Series)
And Betrayer also has a lot on the Word Bearers. (though its primarily about the World Eaters) Same on Know No Fear.
First Heretic, Aurelian, and Betrayer. Know No Fear also features quite a few. The Word Bearers are actually some of the most central characters and appear in many books where they aren't the focus.
You should probably read The First Heretic before anything else in the Horus Heresy anyway, it sets up the rest of the series.
I bear no love for chaos marines, but First Heretic and A Thousand Sons are really great stories, which reveal the tragedy that led to the fall of those legions. I enjoyed them a lot.
regarding the horus heresy time do not forget he Mark of calth anthology; http://wh40k.lexicanum.com/wiki/Mark_of_Calth_(Anthology) and the Purge novella: http://wh40k.lexicanum.com/wiki/The_Purge_(Novella)