If you love classic literature, LibriVox is an incredible resource. It is a volunteer-driven platform that records public domain books. It is entirely free, completely legal, and available as a mobile app. 3. Spotify (Free Tier)
If you love the concept of an organized laboratory of digital books, several highly robust and 100% legal platforms match this exact description. They allow you to scale your library up to thousands of titles for zero dollars. 1. Public Library Connections (Libby & Hoopla) audiobooklabs free