The copyright world owes a debt of thanks to Stanford University for creating a database of copyright renewal records for books published between 1923 and 1963. These dates are significant because anything published before 1923 has fallen into the public domain, while works published after 1963 had their copyrights renewed automatically by the 1976 Copyright Act. That leaves a lot of material in a kind of netherland — assuming the book was published with notice and registered, its copyright had to be renewed (under the earlier U.S. copyright law) after the initial 28 year term in order to have a second 28 year term. If a registration was not renewed (and many were not), the work fell into the public domain; if it was renewed, the work was automatically brought within the ambit of the new law and will be protected until at least 2019.
So it has been very important to know if these mid-century works were renewed or not. Unfortunately, the only Copyright Office records at the Library of Congress that are online are those filed after 1977, so there has been a big gap for which one either needed to search the printed volumes that were published every six months or just give up on knowing for certain. Now it is possible, and much easier, to determine with some precision whether or not many mid-century works are in the public domain and, therefore, freely available for scholarly use, digitization by libraries, etc.
It is interesting that the Google Book Project has treated all post-1923 publications (even government publications that are not subject to copyright protection) as still protected by copyright, giving that project an artifically narrow window on the public domain. Because Google’s scanning work is done so fast and in such volume, it is probably unrealistic to expect them to make fine copyright distinctions. Nevertheless, those distinctions just got a lot easier, and it is to be hoped that Google, or other digitization projects, will use the Stanford database to provided greater access to material that really is the common property of our intellectual heritage.