I love everything on Textkit, but as a suggestion for new material, what about some textbooks on classical language related subjects? Book discussing various Greek and Latin authors, political situations, use of language by Caesar, Cicero etc., ancient history. It’s a big help when reading ancient authors to know something about their context and what was going on at the time they were writing. I have just bought another swag of old books (I think another bookshelf is on the way) and will see if there is anything appropriate there. Just remind me of the publication year prior to which you can legally reprint books in USA - it’s different here in Australia.