You probably know about the following public domain book sources (im guessing they can be read using the Libby app or also Overdrive or other reading apps) - maybe that's where the public library that you're a member of got these books for their system?:

https://openlibrary.org/

https://archive.org/details/books

https://www.gutenberg.org/

https://standardebooks.org/

ETA: I reread your post and saw you mention audiobooks. I believe the Internet Archive (and maybe the other sites I mentioned) has public domain audiobooks also.