Don't read Battle for the Abyss, I could have been reading another HH novel instead of wasting time on that mess - tells you virtually nothing about the Word Bearers past generic evil bad guy and good Ultramarine.
Battle for tge abyss isn't a bad read, it's more like that film you pop on that you kinda like, but you have it on in the background while you do other things.
I despise the Word Bearers legion. Cowards and religious fanatics. They all need put to the chain sword....
*Quietly activates the blood etched runes, lovingly painted on the inside of DaggersEdge's helmet* And now... we wait...
*Emperor Text-to-speech voice activated* Retreating when you are losing a battle is not cowardice. It is common military strategy. Something the Smurfs tend to lack.
I was going to pipe in, but I see you covered it. Calth, plus everything associated with the build up prior to, and the aftermath. Several novels and novella.
Don't forget being the original gangsta... aka First Heretic, backstabbing traitors. (Spoiler Alert) Oh how I hated Erebus reading through the Heresy. I was overjoyed when Lorgar sold him out to Kharne over the event surrounding Argul Tal.