One of the first post-apocalyptic novels, and quite ingenious and gripping, even though written in the 40s or 50s. The agent is biological warfare affecting the vast majority of the population except for a few who are immune), as I recall.

Of course, this means that most of the technology and infrastructure is still intact, although not very realistic by modern standards. For example, hydro power supplies electricity to some communities for months after the event, until maintenance failures bring them down.

Stewart was a trained physical geographer, though, so his description of how the natural world reacts when the hand of man is suddenly withdrawn is very well thought out. His books "Storm" and "Fire" are descriptions of natural phenomena which are still read today. And his account of how the human race responds after the biological attack in both the short and long term is also quite convincing. I wouldn't doubt it would still be worth reading today.