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.
Fire safety, fire prevention, fire fighting. Y’all got the rest covered already. These things were and still are learned the hard way. No reason for us to repeat the experiments with more lives.