I'm looking for a novel set in America after a plague has wiped out nine tenths of the Earth's population. Although dark in theme and incident the novel has a lively comic tone. The central character is an old man, a survivor of the plague. Although a cynical con artist, the old man is too full of humor and good sense to seem bitter.
He's accompanied by a young couple accused of witchcraft (they were reciting love poetry to each other while cuddling). The trio have the job of delivering a microfilm library to a community of archivists in New York. They start out in California.
We're told that many small towns burned every book except the Bible. Fear of knowledge and science are rampant. The group has a run in with a religious zealot called "Simple Simon." The old man cons him with a cure for warts. While dealing with the zealot, they meet a government agent called The Pieman.
Subplots involve a young man exiled from the archivist community for the crime of writing poetry. And the voyage of a Mars colonist to Earth.
The book must have been written after the early 1960s and I read it before 1980. The story wasn't set a full century in the future.
EDIT: (added from OPs comments) When the young couple accused of witchcraft is brought to court, we find out, through the "judge's" internal monologue that he's an actor. The government uses fraud and deception to deal with the more feral communities. There is an off hand mention of the Duke of California. The journey across America is in a medicine show wagon drawn by horses.