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.
Some basic concepts of medication - clean wounds (preferably with alcohol, because it kills germs), understand how germs spread, and know the basic ideas behind vaccines. If society had known these things 1000 years ago, it would’ve saved billions of lives.