The Imperial Library stores a vast range of digital books. You can find all sorts of content on it. At the time of writing, there are over 400,000 books in this online library.
Just be careful of any possible copyright infringement when you access this library (or any other onion site).
Link to the Imperial Library:
http://kx5thpx2olielkihfyo4jgjqfb7zx7wxr3sd4xzt26ochei4m6f7tayd.onion/
