It took me two or three tries, but now I put 2-6 up there with the first book.

I think the reason people have trouble with 2-6 is that they're ultimately a reversal or deconstruction of the Hero myth. The first book is a fairly typical "Rise of the Superhuman Hero" tale, and that's why it's accessible. Book 2-6 explore the dark side of this myth, which is why they're unique, somewhat inaccessible, and really interesting.