I can't help but feel soap making itself wouldn't be as much use as why/when to use it?

Mixing oil with the ashy water (which is as simple as soap's gonna get) is reasonably easy to do and so useful that even without a civilisation people would probably be doing it either through discovery or by keeping doing it?

I think things like "how to build a wooden bridge so it will hold a laden cart and not fall down" are more likely to be lost without civilisation while still being incredibly useful (although I can't say I'd be very good for that)

I might add a section on refrigeration methods like zeers or wind towers/yakhchāls if the civilisation would be somewhere hot and dry, otherwise maybe something on using rivers for powering looms, mills etc.