I am writing a polish math textbook/wiki on neocities. Currently I'm doing basic algebra (here it is in 6th grade). I'm focusing on school math, but might branch out after I cover it more thoroughly. I picked basic algebra because it was my breakthrough to loving mathematics. That's when I started getting passionate.

You can find it here; krolowanauk.neocities.org

Just translate it if you don't know Polish...