The author is really trying to just sell books now instead of trying to put quality work out. The switching back from Grey to Art is pointless.
The real question is why is Arthur still getting his butt kicked? Did he not learn anything from his training with the Asura?
Book 8 is worth listening to. The story at least moves forward a little but author does the switching back and forth again. Seems like author wants to make this series 30 books long. I am sure most of us would prefer quality over quantity.