You can download tons of out-of-copyright books as .pdf files from Google Books.
Some examples:
Ray's Arithmetic, and some of the answer key books
Harvey's Grammar (and an answer key for Practical Grammar)
some really good high school composition and rhetoric books
tons of primers and readers (McGuffey, among others)
History books by Edward Eggleston, James Baldwin, H A Guerber and lots of others
Science/nature books by Jean-Henri Fabre, Parables from Nature, and more