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.
Yeah, that would be crucial too. Antibioitics and the risks of antibiotic resistance need to be included. But to create and purify effective antibiotics, you also need to start with the scientific method, then branch into chemistry, biology, etc. Glassware and procedures to minimize contamination would be important to effectively extract helpful ingredients from potentially harmful molds/other sources.
Depending on the starting scenario, it might be possible to skip much of that at first if we had leftover supplies from a prior civilization. If this site is to be believed, it sounds like making penicillin at home is quite a process, but doable if you’re able to get the right supplies. I don’t see any efficient pathway from here to there if we had to start from zero though.