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.
I’d put the National Electrical Code out there. It doesn’t tell you how to do everything with electricity, like generating it, but it does give a lot of details of how to wire things safely.