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.
“We took all the easy to reach resources. Better get used to being primitive, suckers!”
Thus making it even more accessible, as ores have already been extracted and refine.