Public domain is the mass of intellectual property (books, songs, movies, etc.) that is not protected by copyright, trademark, or patent laws and is available for the public to use freely. Reasons a work is not copyright protected include:
Copyright has expired for all works published in the United States before 1923. As such, we are free to use these works without permission. Due to passage in 1998 of the Sonny Bono Copyright Term Extension Act, no new works will fall into the public domain until 2019 when copyright expires for works published in 1923.
That an individual found something, (e.g. a clinical tool, a book chapter, an episode of a television program) on the Internet does not mean that the person who posted or uploaded the material had the right to do so. Content being used illegally is just that - content that is being used illegally. Do not choose to augment the copyright infringement.
See links compiled on another page of this guide to very informative material from the YouTube Copyright School.
More than 33,000 books in the public domain can be dowloaded at Project Gutenberg.