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.
Creating basic tools, shack building, navigation, breadmaking, pottery. I’m no expert in that, but child bringing and care would also be needed to not lose every first one in infancy. Guess, it’s enough to survive and figure out other things.