Goethe Institut Onleihe: Free digital library of books, audiobooks, magazines, other resources (all in German of course).  VERY worth it to sign up: https://www.goethe.de/ins/us/en/kul/bks/onl.html

This link will take you to the materials the Onleihe has for German Language Learners (you may need to be logged in, however.