Glen Weldon writes about Junot Diaz and superheroes in literature:
Diaz is by no means the first contemporary writer to take funny books seriously; novelists Michael Chabon and Jonathan Lethem have famously trafficked in the uniquely American myth and metaphor of the comic book hero. And lately, writers of nonfiction have begun to explore the early history of comics, unearthing gritty tales of corruption straight out of the pulps. Meanwhile, a new generation of critics is busily deconstructing our enduring fondness for those who mete out swift justice in body-conscious, fashion-forward unitards.
Weldon goes on to list a number of prose books that deal with superheroes, none of which I've read, so I can't speak to their quality or lack of.