A truism in American copyright is that works published in the US prior to 1923 are in the public domain. My copyright chart says it, so does Lolly Gasaway's, and the Google Book Settlement works on this assumption.
By and large this is true. Up until 1998, the longest term that a work could receive was 75 years. That mean that works published in 1922 entered the public domain on 1 January 1998. Later that year the Sonny Bono Copyright Term Extension Act added twenty years to the term of works that were still protected by copyright, but it did not restore copyright in works that had already entered the public domain.
But things are never clear in American copyright. I was re-reading Robert Brauneis's justly-praised essay, "Copyright and the World's Most Popular Song." In it, Brauneis argues that "Happy Birthday," which generates perhaps $2 million a year in licensing fees, is actually in the public domain. In order to make his case, he undertakes a thorough review of copyright under the 1909 Copyright Act. Brauneis notes almost in an aside that "Happy Birthday" was published in 1912 in "The Beginners’ Book of Songs" and again in 1915 in "The Golden Book of Favorite Songs." A library might think that since these editions predate 1923, they could be digitized with impunity.
According to the current owners of the presumed copyright in "Happy Birthday," however, these early publications were unauthorized. They argue that the first authorized publication of the lyrics to "Happy Birthday" occurred in 1935, and copyright runs from that date. Digitizing either the 1912 or 1915 volumes would therefore infringe on the copyright first secured in 1935.
As a practical matter, there would likely be little risk in digitizing any pre-1923 edition of a work. The presumed copyright owner would have to establish with certainty that an authorized publication occurred at a later date. As time passes, it becomes harder and harder to document authorship of earlier works, and so the likelihood of a complaint or suit diminishes. But there is a big difference between saying "this is in the public domain" and "this may be copyrighted, but there is little chance that I wll be sued."
In short, just because a work was published in the US prior to 1923 does not mean it is in the public domain. The first authorized publication needs to have occurred before that date. This is just one more example of how hard it is to establish with certainty the copyright status of a work.