Out of coyright books and publications can be a good source of free content for your websites and blogs. The first thing that you need to know is what books are out of copyright. Second, you may ask yourself where can I find out of copyright content. Third, there's the question of "out of copyright content" and duplicate content.

1) What Books are out of Copyright?
Books published prior to 1923 are in the public domain and may be freely used. Additionally, books published 1923 through 1963 with notice but copyright was not renewed are in the public domain. However, it may be quite difficult to establish which books from 1923 through 1963 are out of copyright. This is due to changes of ownership and many of the original companies are out of business.

Another thing that you should know about is the "Mickey Mouse Act" or The Copyright Term Extension Act (CTEA) of 1998 whic extended copyright terms in the United States by 20 years.

The Copyright Term Extension Act (CTEA) of 1998 extended copyright terms in the United States by 20 years. Since the Copyright Act of 1976, copyright would last for the life of the author plus 50 years, or 75 years for a work of corporate authorship. The Act extended these terms to life of the author plus 70 years and for works of corporate authorship to 120 years after creation or 95 years after publication, whichever endpoint is earlier.[1] Copyright protection for works published prior to January 1, 1978, was increased by 20 years to a total of 95 years from their publication date.
This law, also known as the Sonny Bono Copyright Term Extension Act, Sonny Bono Act, or pejoratively as the Mickey Mouse Protection Act,[2] effectively "froze" the advancement date of the public domain in the United States for works covered by the older fixed term copyright rules. Under this Act, additional works made in 1923 or afterwards that were still copyrighted in 1998 will not enter the public domain until 2019 or afterward (depending on the date of the product) unless the owner of the copyright releases them into the public domain prior to that or if the copyright gets extended again. Unlike copyright extension legislation in the European Union, the Sonny Bono Act did not revive copyrights that had already expired. The Act did extend the terms of protection set for works that were already copyrighted, and is retroactive in that sense. However, works created before January 1, 1978, but not published or registered for copyright until recently are addressed in a special section (17 U.S.C. § 303) and may remain protected until the end of 2047. The Act became Pub.L. 105-298 on October 27, 1998.
see http://en.wikipedia.org/wiki/Copyright_Term_Extension_Act

You may also find more information on what is in the public domain @ http://www.copyright.cornell.edu/resources/publicdomain.cfm

2) Where can I find out of copyright content?
You can download books that are in the public domain from Google Books. Public Libraries may also be a good source of finding content in the public domain. Most libraries get rid of books that aren't used much. So they may not have too many old books on the shelves. But, you should be able to find plenty of old publications on microfilm. You can usually blowback the film copy to a printer for a small fee. Then, all you need to do is to scan and ocr the copies to a text file. Other good places to look for old books are garage sales, church flea markets, estate sales and old book shops.

3) Out of Copyright Content and Duplicate Content
Any public domain books that you are able to download on the web, most likely, will have duplicate content issues. Publications from Google Books will have the same issues as they have already been indexed by Google. So what public domain content can I use? Well, first of you can rewrite the works that you find on the web. However, the best sources for non-indexed content will be from out of print books and articles on microfilm. You can check a sample on copyscape before you invest the time in converting the book to editable format.

Many of the books that are in the public domain are "out-dated". But you may find that travel articles, recipies, how to books i.e. building, woodworking etc and fiction can all be used as valuable content on your website.