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 All Mighty Guardian of the Book, Keeper of Knowledge.
You will also be required to know it by memory cover to cover and read it on request for anyone.
Then my knowledge of my own limits must. Be my contribution to the book.