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.
The sign of a ball with three rays shall be repulsive for thee, thou shalt not enter any shelter nor edifice with the sign of a ball with three rays, nor destroy spoken buildings.
And to react to @bool@lemm.ee I would probably be able to write some primitive method of computing the logarithms.