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.
Does it count if I have been keeping a copy of all the Kiwix releases and could submit those documents? It would certainly be one hell of a kickstart.
Otherwise, screw civilization… Here’s how you build and tune a trebuchet (yeah I have a bit of experience with the construction, as long as someone else supplies the rope).