Amazon has a huge offer of free ebooks.

thank you ✨

Take a look at https://www.gutenberg.org/browse/languages/es for written books, and https://librivox.org/search?primary_key=5&search_category=language&search_page=1&search_form=get_results for audio books All of them are without copyright; so they are old books; but some of them are very good.