In any case everything we know about modern history was penned by either eyewitnesses, or people who have researched the events and penned their own accounts. The Bible is no different, except that God has guided the recording process, to keep it error free. Just because we don't have the reference materials that Moses used, does not mean they did not exist!
Who wrote Shakespears plays? We don't have the original manuscripts, so perhaps they were handed down by oral tradition as well.
http://search.live.com/results.aspx?q=Who+wrote+Shakespeare%27s+plays&go=&form=QBLH
That is a matter of some debate.