Books in the public domain are free for anyone to use. One way authors can use these stories are as the basis for their own books. An example of a publisher who does this well is Disney, which has produced movies based on stories by the Grimms’ Fairy Tales, Mark Twain, Jules Verne, Aesop’s Fables, folk tales, Greek myths, legends, and more.
Public domain stories can be retold, remixed, expanded upon, or told from a different angle or point of view. (For fun, I started writing my own version of the story, The 12 Dancing Princesses on Wattpad.)
With that in mind, here is a list of places where you can read and/or download books in the public domain: