This is without doubt a great book and goes into the favourites section of my Heresy collection alongside Horus Rising, Legion and Mechanicum. It's the first Heresy book in a while that actually moves the series forward despite its events remaining before Istvaan.
Graham McNeill really defines the 'Sorcerer Astartes', fleshing out these very different Marines and orchestrating the most believable and moving fall of a Legion to date; casting a shadow of doubt over my own staunch loyalist views *shudder* somebody call the Inquisition!
He makes a sterling effort to forge coherency amongst the broken and confusing cannon of the 40k verse (at one point openly questioning and including in the story arc one of it's contradictions) with many questions that have niggled me for years finally answered and some great hints dropped about others.
Some faults, McNeill still loses himself occasionally (although he is getting better) in overblown and unnecessary description, how many times we need to hear how totally awesome a Primarch is, or how the Son's control and balance their powers, we hear you we get it, please stop repeating yourself Graham. All this does is slow down the flow of what is a captivating story and fills up extra pages, are you trying to meet a certain word count?
Also I lost track of how many times he used the word Impossibly... something was Impossibly strong, Impossibly bright, Impossibly Impossible!!! Really grated and I found myself cursing out loud every time I bumped over one, there must be host of better words for describing an amazing power or technical feat.
Other than these minor quibbles (what do I know, I'm not a bestselling author) this is an awesome read and I highly recommend it.