Mechanicum is good and I agree that BFTA was not the greatest book and I did have to struggle through it I liked how the ending cut off
. I'm pumped and awaiting the continuation of the actual Horus storyline, can't wait to see how the siege of Terra is shown.
While it would be badass to know what happened to the missing chapters I don't think
GW will ever tell. I read an interview with a
GW employee regarding canon (which I'll link if I can find it again) he said that nothing in the
WH40K universe is to be considered canon. Citing that all stories may be told from one side or that all the facts may not be included because the "author" doesn't know them. This is intentional so players can create or do anything within the
WH40K universe. I was pretty disheartened after reading that, I'll try to look it up and get it here.