I just can't stop reading the Horus Heresy books, having recently finished the third book, Galaxy in Flames. In it, the Warmaster Horus is already corrupted and plans an attack to Isstvan III, in theory to crush some humans who are fanatics. But there's more than meets the eye, as the attack seems too big for just some rebels and not every space marine is going to drop to the planet surface to fight...
At this book the "new" chaotic Horus reveals himself (and those loyal to him) and the first space marine vs space marine fights happen. The story arc progresses and there are interesting events, although I felt that the book extends a bit too much certain fragments and the main theme progresses slower than in past books. In any case, interesting reading, especially as the four chaotic powers start to shape.