It seems like the general idea for the full story was done, and "The Book" (as he called it) "existed" in a very rough draft, but with each release he's going back, re-editing and re-working them, adding bits here and there, improving the story until it's to the point that he wants it to be. It would make sense to me that, with each book, the ripple of stuff that needs fixing in the story later on needs more and more re-work.

I believe the first two books were released 4 years apart. It seems that that's the cadence for the books. And hey, Auri novella this fall! :D