The Centurion's Empire by Sean McMullen

In the year that Mount Vesuvius destroyed Pompeii, the Roman Centurion Vitellan set off for the twenty-first century as Imperial Rome's last human-powered time machine. He killed an unfaithful lover by just letting her grow old, but her hate pursued him across seven centuries. In 1358 he stood with a few dozen knights against an army of nine thousand to defend the life of a beautiful countess...and earned a love that would conquer death.
Now Vitellan has awakened in the twenty-first century, a bewildered fugitive, betrayed and hunted in a world where minds and bodies are swapped and memories are bought, sold, and read like books. But worst of all, a deadly enemy from the fourteenth century is still very much alive--and closing in.
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