I only went to Wikipedia after I posed this question. I honestly did not know that he had done prequels and side stories. I thought they were all followups. Also he was pushing out one a year? That can't be good. I might start with the last ones assuming he's found his stride by then. So library it is.
eta: I also consider the last two books to be the weakest. He could have ended it after GEoD and it would have been a solid series.