Have you considered Boy Scout Merit Badge Pamphlets? They're short little how-to books for children. You can find lots online for cheap for nearly anything like mining to archeology. The new ones aren't nearly as good but they're still sold today for less than 10 dollars. Ebay prices vary but should still be cheap for physical literature if that's what you're wanting.

https://www.scoutshop.org/literature/program-literature/merit-badge-pamphlets.html