I’m always excited when I find open textbooks for K-12. At my kid’s school, textbooks and manuals run in the $100+/copy range, and if you calculate the cost of that to a school, a district, and then to an entire province the numbers are astronomical. Of course, this is the kind of math you do when a sector has been gutted for 10+ years and kids with learning disabilities or needing extra support can really only get it if the parents have the means to pay. So the social impact of publisher gouging on K-12 books is significant at any kind of scale.
Finland has made North American headlines over the past couple of years for their apparently excellent K-12 education system, so finding a collection of open textbooks for intermediate and high school math is nothing short of exciting. Instead of linking to just one, I’m linking to the page for the whole collection. Each one is a large pdf, which Google won’t automatically translate, so you have to do a cut and paste job if you are curious.