There's an essay in Castle of Days where Gene Wolfe wrote about his writing process. He would rewrite the manuscript for his books from scratch multiple times so that he could incorporate what he wrote later into the beginning of the book. I think he would end up completely rewriting the book up to 4 times. That's why there's so much to get out of them on a reread.

And that sounds crazily presumptuous, but when I finished my exact thought was "I have to read that again."