Restricting access to multiple file directories

I have recently purchased the pro version of s2Member, and am trying to set up restricted access, only available to paid subscribers, to the html versions of multilingual interactive picture ebooks for young language learners. These are neither pages nor posts; each ebook is a web app, comprised of file directories that are housed in the public html directory of my WP site. There are currently 18 books, but eventually, there will be multiple hundreds of titles. Any clues on how I can restrict access to these? I also have other parts of the site that needs to remain unrestricted; the home page, an about page, a page showing the different books available (but not giving access to the books themselves), a meet the team page, etc.