A number of copy-protection systems for mid-80's to mid-90's computer games asked that you opened the printed manual that came with the game and go to—e.g.—page 32 and type in the first word of the third paragraph. Doesn't seem that hard to automate doing that for ebooks, these days. After all, Goodreads is owned by Amazon so they have the corpus. Just use chapters instead of pages; that will work across all formats.