A handy chart I had not yet seen, from the Cornell Copyright Information Center.
Footnote 7 is useful:
A 1961 Copyright Office study found that fewer than 15% of all registered copyrights were renewed. For books, the figure was even lower: 7%. See Barbara Ringer, “Study No. 31: Renewal of Copyright” (1960), reprinted in Library of Congress Copyright Office. Copyright law revision: Studies prepared for the Subcommittee on Patents, Trademarks, and Copyrights of the Committee on the Judiciary, United States Senate, Eighty-sixth Congress, first [-second] session. (Washington: U. S. Govt. Print. Off, 1961), p. 220. A good guide to investigating the copyright and renewal status of published work is Samuel Demas and Jennie L. Brogdon, “Determining Copyright Status for Preservation and Access: Defining Reasonable Effort,” Library Resources and Technical Services 41:4 (October, 1997): 323-334. See also Library of Congress Copyright Office, How to investigate the copyright status of a work. Circular 22. [Washington, D.C.: Library of Congress, Copyright Office, 2004]. The Online Books Page FAQ, especially “How Can I Tell Whether a Book Can Go Online?” and “How Can I Tell Whether a Copyright Was Renewed?“, is also very helpful.