Like with Book 2, I bought Galaxy in Flames immediately after finishing False Gods (review here) and started reading just about straight-away. This novel more or less ties up the first trilogy that started with Horus Rising (review here). As with previous reviews, I'll try to avoid spoilers, but this one might make it tougher; at this point there's no way to avoid mentioning some events of Book 1 and 2, so if you haven't read them yet, maybe wait until you have.
This book is a little more action packed than previous versions, which I somewhat expected from Ben Counter (Soul Drinker, while maybe over the top, has a special place in my collection). There is also the ever-present impending and inexorable doom that comes with any tragedy; from the beginning there is no real secret to the fact that the heroes of the series thus far will be betrayed, and most will probably not survive.
The plot of the novel was more confined than the previous novels, but more depth was given to the characters I felt, particularly the non-astartes. That could also have been because of a need to balance the combat scenes with scenes of intrigue and clever exposition.
I think the part I liked about Soul Drinker was Counter's ability to use combat and the lulls in action to develop characters. I was not disappointed in this, particularly in regards to Saul Tarvitz, who was really expanded on after his brief cameo in Horus Rising, though the argument could be made that he is more or less a purple-armored Loken.
I was relatively pleased with the way Horus was handled in this book in comparison to in False Gods - more mystery that still made sense - if that makes any sense at all. The decision to turn was already made and no longer a point of contention; instead Counter gave glimpses to his flirtation with the Chaos powers, how deeply Chaos had infiltrated the Legions represented, and Horus's ruthlessness and tactical acumen.
Finally, I was satisfied with the ending. Things don't turn out the way you would like, of course, but that's not a surprise. The way the last stands and endings were handled was a little stirring, particularly Torgaddon and Aximand's confrontation, which stole the show from Loken against Abbadon in my mind.
On finishing the novel, I was interested to note that the subsequent novels in the series take a step back from the timeline of the first three, either beginning during Galaxy in Flames, in the case of Flight of the Eisenstein, or well before. I'm looking forward to learning more about other Legions as the series continues.