Having read "The Outcast Dead" over the vacation period, I had a number of criticisms and questions arising. For those reader who haven't read the book, please turn away now as I don't want to accidentally be giving spoilers!
My first criticism about the book is the names of the characters and their spelling. One of World Eaters characters is called Asubha (at least according to the dramatis personae at the front of the book; which curiously enough translates as "impurity" in English). However, at various points in the book, the spelling shifts to Ashuba. I personally found this to be quite distracting and hoped that it didn't mean a lack of proof reading. Yet, there were other gramatical mistakes that a read-through should have caught (e.g. repition of "the" -- i.e. "the the") and some sentences that didn't feel quite right.
Those criticisms aside, I did enjoy the book and felt it lit up a previously unexplored region of the Horus Heresy' parameter space.
But I was left wondering at the end of the book about the timing of various events. In particular, when was Magnus' warning to the Emperor delivered (at least in relation to real-time events in the Istvaan / Isstvan system)? My impression was that Magnus knew things before the Heresy started to unfold and reveal itself fully. But reading the Outcast Dead, I'm now a bit confused about whether the drop site massacre happened before or after Magnus delivered his warning. And following on from that, how soon were the Space Wolves deployed to Prospero? Or put another way: why weren't they dispatched to Isstvan with the Iron Hands et al. or told to come back to Terra to help defend like the White Scars were? So confused!