I'm only up to page 25, but already I am completely engrossed in the book, thanks to the inclusion of pretty much every major Iron Warriors character from the Ultramarines Omnibus and the Iron Warriors Omnibus. It is very cool to read about them in the original heresy setting (pre-warp exposure).
I especially like the reference to Warsmith Dantioch (the only warsmith to refuse Perturabo's orders to join against the Emperor).
However, I feel as if I have a read a short story on the Angel Exterminatus somewhere before, something to do with a device called a 'pylon' that was secreted away on a Mechanicus forge asteroid, that the Iron Warriors went to go steal. Any thoughts on that?

But I digress. The novel itself is fantastic so far.