Knowledge is only as good as the people who understand it and retain it, and this will never change. We can preserve our knowledge in books but that is only meaningful if someone can come along, read the book, and apply the knowledge. If every neurosurgeon in the world died tomorrow, in some horrible World Neurosurgery Conference volcano/cyclone/food poisoning disaster, while we do still have all the neurosurgery knowledge in books, we can't still say that we, as a society, know how to do neurosurgery. Someone has to re-learn it.
Mokalus of Borg
PS - Regaining that knowledge becomes even harder if it's written in a dead language.
PPS - Dead languages are, themselves, excellent examples of this lost knowledge idea.