An updated list of sites that offer free public domain books in electronic and audio format.
Every year new publications enter public domain. That means their intellectual property rights have expired or are not applicable any longer.
The content of these works becomes available for public use. Anyone is free to use it – but also to reuse it, for instance publish a new edition. Therefore you may find in major ebookstores (Kindle, Nook, Kobo, iBook Store, or Google Play Books) public domain books that are not free.