This is an answer I've given before, because I think it's one of the great tragedies of the classical world:
I imagine the "Dark Ages" would have turned out quite differently had the libraries of Egypt, Rome, and all the other great ancient cities not burned. So much knowledge -- especially in the sciences -- was lost, setting human progress in the West back several centuries. If I had a time machine... I would definitely go back to save the second volume of Aristotle's Poetics, and Aristarchus's text on astronomy, and Ovid's Medea, and Euripides's lost plays...
