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.
I read it as “what do you have enough knowledge of that you could aid in the birth/advancement of a new civilization?” Doesn’t matter if you have it fully figured out, just what knowledge can you provide that would be highly valued if all knowledge were otherwise lost.