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Actually, isn't that just one book?
"The Deadly Mission of Phineas Snodgrass"
The wikipedia article on "Lest Darkenss Falls" states:
"A response to this story is "The Deadly Mission of Phineas Snodgrass" by Frederick Pohl, in which a man travels back to 1 BCE and teaches modern medicine, causing a population explosion. It ends with the fantastically overpopulated alternate timeline sending someone back to assassinate the title character, allowing darkness to fall for thankful billions."
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