College & Research Libraries has released an e-print of "How Large is the 'Public Domain'?: A Comparative Analysis of Ringer's 1961 Copyright Renewal Study and HathiTrust CRMS Data" by John P. Wilkin.
Here's an excerpt:
The 1961 Copyright Office study on renewals, authored by Barbara Ringer, has cast an outsized influence on discussions of the U.S. 1923-1963 public domain. As more concrete data emerges from initiatives such as the large-scale determination process in the Copyright Review Management System project, questions are raised about the reliability or meaning of the Ringer data. A closer examination of both the Ringer study and CRMS data demonstrates fundamental misunderstandings and misrepresentations of the Ringer data, as well as possible methodological issues. Estimates of the size of the corpus of public domain books published in the U.S. from 1923-1963 have been inflated by problematic assumptions, and we should be able to correct mistaken conclusions with reasonable effort.