Inferno — Larry Niven and Jerry Pournelle
I read this when it was first serialized in Galaxy magazine, a long time ago. Niven and Pournelle just wrote a sequel, so I picked up copies of both. It's about a science fiction writer named Allan Carpentier who manages to get himself killed by completely avoidable accident at a con. (Alcohol was involved.) He winds up in Hell, just as Dante describes it. Of course, being an SF writer, be doesn't believe it for a second, He thinks the place is some sort of elaborate Riverworld like artifact created by some future (or possibly alien) intelligence for God knows what reason. He calls it Infernoland.
Carpentier meets a man who says there's a way out, and who offers to guide him there. The way out is allegedly in the very center of Hell, so off they go, traversing the nine circles. They meet people, some famous (William Bonney), some not, and some unnamed (in-jokes of varying obscurity). (They pass the tomb of an atheist with a neon sign on it that flashes "So it goes," and Carpentier mentions that the occupant denied that he wrote SF, too.) There are new interpretations of old sins, and some sins that Dante never thought of. (Niven and Pournelle's God is very libertarian in outlook.)
Eventually Carpentier comes to realize that he really is in Hell, which raises a question for him. Why is God so vengeful? As he notes at one point, "We're in the hands of infinite power and infinite sadism." As they travel further, though, he begins to realize that it's not God that's keeping the damned souls in their torment, but the souls themselves. Hell is the last test, and it is possible to escape.
The book has aged a little. One of the first people Carpentier meets is there for a sin that was big news in the seventies, but much less relevant these days. Actually, it's not the sin that's irrelevant (lying by twisting scientific results to suit one's own agenda), but rather that the actual subject (the banning of cyclamates in the US) is now mostly forgotten. It's also occasionally annoying to see some of the things that Niven and Pournelle seem to view as sins (there are a lot of lawyers in Hell), but it doesn't detract too much from the story. I'm looking forward to reading the sequel.