Updating this, since I last posted here, i have read the first 6 books of the series, and I am on book 7 at the moment.
The first 3 books of the series are a trilogy in themselves, focussing on a captain of the Luna Wolves/Sons of Horus Space Marine Legion.
Book 1 - Horus Rising
A fantastic read, really sets the scene for the rest of the saga perfectly, and actually makes Horus a quite likeable character, which makes it all the more shocking when her turns to Chaos.
Book 2 - False Gods
The start of Horus' corruption. It felt a little slower than Horus Rising, and took me slightly longer to get through, but was still very good.
Book 3 - Galaxy In Flames
Brilliant. The full extent of Horus' betrayal is revealed, and the Luna Wolves, as well as many other Space Marine legions are torn apart by the heresy. Couldn't put this one down.
Book 4 - Flight of the Eisenstien
Another very good book, and for you WH40K buffs out there, this book will sow the seeds that lead to the birth of a very important organization within the Imperium.
Book 5 - Fulgrim
Genius. Sheer genius. The longest so far of the books, Fulgrim tells the story of how Fulgrim, one of the brother Primarchs of Horus, is corrupted by both Chaos and his brother. The god of Chaos he bows to is the god of pleasure and debauchery, and as such there is some very disturbing imagery in the book, which I found perfectly fitted in with the story
Book 6 - Descent of Angels
The first book in the series that disappointed me. Not that it wasn't a good book, but it just doesn't tie in with any of what has taken place at all in the previous books. It follows the story of how the Dark Angels legion of Space Marines came to be, and how their Primarch, Lion El'Jonson was discovered by the Emperor. I hope they go back and tie this novel in with another story at some point, and make this book worthwhile.
Book 7 - Legion
Currently reading this. Started very well so far. It is the story of how the enigmatic final legion, the Alpha Legion, turned to Chaos during the Heresy.