SciFi and Fantasy Book Club discussion

I read a fantasy/scifi book a few years ago in which the main charcter, a guy I believe, was a postman delievering stuff in the galaxy I think. Then he found some giant colorful crystals in which there are humans in there. The magic in the story is that there are generations of superheros and the younger the generation of superheros, the weaker their powers. I remeber the story being something among these lines but im not too clear. Could someone please tell me the name of the book.