The First Heretic by Aaron Dembski-Bowden
Lets start by saying that I loved this book. It really got into mankinds first introduction with the powers of Chaos, and it's supposed fall from grace that leads to rebellion. In fact after reading this book, you have to wonder, if Horus had won (sorry for the spoiler) would mankind of ascended. Is the way of the Chaos gods the greatest path for mankind?
Even asking those questions means that the book was well written. I read the book in only 5 days. I imagine writing the book, it was very difficult to engage these concepts well enough, that would allow a primarch to go choose this route. The author not only handled them well, it drove the book and kept me deeply engaged.
Given who the Word Bearers are, it was inevitable that they were going to go this route. What I find disconcerting about mankinds current spot in the 40k universe, is that they now worship the Emperor. If this had been allowed from the beginning, could the Heresy of been avoided? The Word Bearers were the chapter of unquestioning faith and loyalty, how could they fall?
If you only read Dan Abnett's books, I highly suggest you expand the horizon. I had always looked at the Horus Heresy about power. I was wrong. Read this book. Good writing, great story, well delivered.