What is meant by ‘the public domain’?
“Public domain” refers to works in which copyright has expired or where the copyright owner has made a clear declaration that they will not assert copyright in the work.
For example, although the copyright in Shakespeare’s plays expired long ago, many of the published editions of his plays contain added original materials (such as footnotes, prefaces etc.) which are copyright protected because the authors have used skill and judgment in creating the new material. This creates a new copyright in the added original material, but not in the underlying text of the original work in which the copyright had expired.
Project Gutenberg is a good resource for finding literary works in the public domain.