I can’t find a scan anywhere of anything 4a or higher, but if you are willing to pay for a physical copy there are a few sellers on AbeBooks selling “Reach Higher Student’s Book 4A”.
For free textbooks outside of that series:
BCcampus Open Publishing: They provide a ton of free courses and textbooks on a variety of topics. This isn’t really piracy, but it’s got a ton of good information.
I did find one site that seems to have all the PDFs for sale, but I can’t tell if it’s a scam or not. I would recommend looking at some of the free resources available on BCcampus before attempting this.
This is true, and I’m not against “alternative” sources for ebooks (having used them myself), but it’s always better for the libraries and authors if there’s foot traffic to the libraries, since that drives tax dollars which in turn makes its way back to the authors. It’s less than pennies on the dollar, but it’s important.