Hathi Trust is a digital repository library. The repository contains items currently covered under copyright law and items that are in the public domain. While items still covered by copyright law are not fully available to everyone, items determined to be in the public domain are freely available for anyone to view and sometimes to print. Some titles will allow a full PDF download. If you do this, keep in mind you may be downloading a very large file. In a recent Charleston Advisor interview, John P. Wilkin, executive director of Hathi Trust, estimated that between the years of 1923-1963 about 60 percent of works they have reviewed are in the public domain. He also states that many items in the collection and published after 1923 are in the public domain because they are U. S. government publications.
Some publications are available for sale through print on demand agreements made by partners within Hathi Trust. There is a “find in a library” feature that uses the OCLC Worldcat database to help you discover which libraries have copies of the book available to lend. This resource may not be your first stop in the research process, but it may be worth a look.