Books in the public domain (cc0) are copyright-free literary and artistic works. They can be copied, remixed, and redistributed without the need for permission. Public domain books are also available in digital formats that are easier to find than physical copies of the same book.
Most of the public domain books can be accessed online at Project Gutenberg or Wikibooks. While there are some websites that offer them as pdf downloads, such as the Internet Archive. These online resources have made it possible to find and download public domain books easily and legally.
This page contains links to sites that have books and book collections that are in the public domain. Other pages of interest may include Digital Collections: Academic, & Magazines & Journals