Ok, I'm been trying to identify this book for about a couple of years now (on and off). About ten years ago (when I was just entering high school) I read this book by either Ben Bova or Orson Scott Card (or could be anyone but for some reason their names come to mind). It's about this guy who travels back in time, to different time periods, ie one of the time periods was the Mongol invasion of Europe. He kind of plays a role in each time period he goes back in as he tries to find his way back to his own time. Does this sound familiar at all??? It's driving me crazy trying to find this book
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Memnoch - Custodian of the Order of the Holy Flame 