Hank Morgan, A Connecticut Yankee in King Arthur's CourtRead about the other entries on the list.
I'm guessing Mark Twain hadn't heard of "the butterfly effect" when he wrote his classic A Connecticut Yankee in King Arthur's Court, because when Hank Morgan wakes up in ancient England, he does the equivalent of murdering every goddamn butterfly he sees. He immediately uses his knowledge of a solar eclipse to convince everybody he's a wizard, and becomes the second most powerful man in the kingdom. He shoots knights with his revolver, blows up Merlin's tower with explosives, creates bicycles, gets King Arthur arrested for slavery, starts a war with the Catholic church, and basically messes with every single person and thing he finds in 528 AD. Where's a timecop when you need one?
A Connecticut Yankee in King Arthur's Court is on Michael Brooks's top ten list of time travel books; it's also the book Roman Simic most likes to re-read.
--Marshal Zeringue