The speculation is that Garro - Oath of Moment is going to be another audio book based on the squareness of the image. I believe that the cover art for The First Heretic has been floating around for a while now.
I have to admit that I have fallen way behind on my Horus Heresy novels. I really enjoyed them when I was reading them and I'm not even sure why I stopped. I had just started the Flight of the Eisenstein when I shelved the series for a while. I think it is probably time to go back. I have a ton of catching up to do.