For many years, genealogists have believed that all books published in the U.S. prior to 1923 are now public domain, meaning those books can legally be copied and sold. Anything published in 1923 or later might be under copyright. The keyword here is “might.” The subject became a bit complicated starting in 1923. I wrote about that in an earlier Plus Edition article that is still available at: http://eogn.com/wp/?p=41410. (A Plus Edition user name and password is required in order to read that article.)
Now the folks at the highly-respected Internet Archive have made a claim that Section 108h of the U.S. Copyright laws are even less restrictive, at least for libraries. That may not be the same as for private individuals, however. Here is a brief quote from the statement:

“The Internet Archive is now leveraging a little known, and perhaps never used, provision of US copyright law, Section 108h, which allows libraries to scan and make available materials published 1923 to 1941 if they are not being actively sold. Elizabeth Townsend Gard, a copyright scholar at Tulane University calls this ‘Library Public Domain.’ She and her students helped bring the first scanned books of this era available online in a collection named for the author of the bill making this necessary: The Sonny Bono Memorial Collection. Thousands more books will be added in the near future as we automate. We hope this will encourage libraries that have been reticent to scan beyond 1923 to start mass scanning their books and other works, at least up to 1942.”
Again, this exemption from the extension of copyright is only for libraries and only for works that are not actively for sale.
You can read the full story in the Internet Archive Blog at: http://blog.archive.org/2017/10/10/books-from-1923-to-1941-now-liberated.