The title is a bit over dramatic but, per the title, if you could contribute with one piece of knowledge to a book that every single individual should learn from in order to kickstart a civilization, what would be yours?
My personal choice would be the process of soap making, from scratch.
They didn’t stop working somehow. The trick is that they need a very high (deadly) bias voltage to work, are mechanically delicate, have to be heated and possibly cleared of gasses leaking in, and from what I can tell have inferior characteristics for a lot of applications.
On the other hand, your relatives are right about the electrical toughness, and they have no firm upper frequency limit, so they still have industrial niches.