Here's a fine summary of Gene Wolfe and his work by Jeremy L. C. Jones (as the intro to his interview with Wolfe):
Gene Wolfe will tell you the truth, in conversation and in fiction, whether you want to hear it or not. He is perhaps best known for his novels set on Urth, including the four-part Book of the New Sun and the four part Book of the Long Sun. He writes fantasy and science fiction, and a grim blend of both that he insists is not horror.
Overall, he writes a story the way it needs to be written and he does so in rich, textured prose that delights on a first and rewards on a second reading.
"The high quality of Gene's prose guaranteed him critical attention," said David Drake, a long-time friend of Wolfe's and author of The Lord of the Isles series and the Hammer Slammers series. "He's also a commercial success, though, which is a very different thing. He's seen enough of life to be able to write about hard places in a fashion to both [speak] to those who've been in them, and to make them vivid to luckier people who haven't personally seen what Gene has."
It's those "hard places" and unacceptable truths that inform even the lightest of Wolfe's narratives. His stories, long and short, can be relentless, brutal, grim, and, word for word, maddeningly beautiful.
'How could the Outsider have chosen such a bungler? ...When had he ever offered a single sacrifice, however small, to the Outsider? Never! Not one in his entire life. Yet the Outsider had extended infinite credit to him... Certainly he would never be able to repay the Outsider for the knowledge and the honor, no matter how hard or how long he tried.' (Gene Wolfe, Nightside The Long Sun)