Here’s a website that you’ll want to bookmark, appropriately titled Books Should Be Free.
From its self-description:
BooksShouldBeFree makes the world’s public domain audio books available for browsing in a visual and entertaining way. . . . BooksShouldBeFree provides a primarily visual browsing experience so you don’t have to read tedious amounts to find listenting materials. . . . All audio books on BooksShouldBeFree.com are in the public domain. This means that no one holds a copyright on these books and therefore anyone including BooksShouldBeFree.com is free to distribute them.
Genres include children’s, mystery, history, literature, philosophy, poetry, and religion. Start browsing here.