From the announcement:
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.
Read the full announcement here.