I've been reading these since book 4 (I started with book 1, but book four was out). He runs through a number of changes in the situation and evolves the setup reasonably well, particularly introducing new long-term villains. He does a little bit of retconning as he expands the universe, but less than most other long-running series, I think. Book 2 is OK, but I think it's book 3 where he really gets going. Oh, and there's a few Chekov's Guns in the first book that he pulls the trigger on after book 10 (I don't want to give spoilers).