Extremely old (historical) books whose contents were recently known but which were subsequently lost and/or destroyed?

Futurist

Ad Honoris
May 2014
22,239
SoCal
Which extremely old (historical) books are there whose contents were recently known (as in, people had access to these books relatively recently) but which were subsequently lost and/or destroyed--thus ensuring that we don't actually have the full text of these books right now?