Works in the Public Domain
Works are deemed to be in the 'public domain' when the copyright has expired, or where the copyright owner has made a clear declaration that he/she will not assert copyright in the work.
However, even a work with an obviously expired copyright, such a Shakespeare play, may contain added original materials, such as commentaries and footnotes, which are copyright protected.
Works available on the Internet are no exception to copyright rules. Most are copyright protected and not in the public domain. When using online materials you should make sure your purpose falls within the educational use of the Internet or fair dealing provisions. If the online content is encoded or encrypted or clearly states that educators cannot use it, then permissions must be sought.
Some sites listing public domain books:
Project Gutenberg Canada