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.
Remind everyone that there is no one true religion because there never was one before, there isn’t one now and there never will be be one.
“Yes, all the religions before were false, but ours is the right one!”