The Deliverance Lost book about the Corax and the Raven Guard is pretty much 50/50 Raven Guard/Alpha Legion. Also tells a lot of the story from one of the Alpha Legionnaires who infiltrated the RG. Not bad for a W40K novel.
One of the big problems with the HH series is that we're told that it was unimaginable for a Space Marine to raise his hand against another Space Marine, much less a Primarch attack another Primarch. Yet even before the days of the Istvaan drop site massacre, the Primarchs behave like bickering children, threatening to kill each other at every other meeting. Same with the Marines. Yet every six pages, there's one of the Emprah's finest getting ready to throw copper at another one.
The Horus Heresy stuff should have been left alone, myths and legends lost through time.