US Copyright's "Weird Window"
US copyright terms are more complex than they should be-everybody seems to agree on that but Big Media. Here's a nice short summary that I have presented before. What's interesting is what happens in a sort of weird window between 1923 and 1963. Books published in that window bearing a legal copyright notice may or may not still be within copyright. The key is whether the copyright was explicitly renewed by the rightsholder. No renewal, and the book passed into the public domain after its initial 28 years of copyright, which would be no later than 1991.
Most books from that period that we even moderately successful financially have been renewed, but I've found a fair number of reasonably interesting books that were not. Most of the books I used in my researches into the fourth dimension in high school were either pre-1923 or never renewed: Coxeter's Regular Polytopes, Manning's Geometry of the Fourth Dimension and The Fourth Dimension Simply Explained, Somerville's An Introduction to the Geometry of N Dimensions. All are now in the public domain, and all are available from (surprise!) Dover Books in print editions, but I would certainly like to see them become nicely reset PDFs and not simply holographs. (My copy of Coxeter fell apart back in 1970.)
A lot of old electronics and amateur radio books were never renewed. All the Frank C. Jones amateur radio books that I have (great tube-era construction stuff!) have expired, and they were beautifully done. The late Don Stoner's New Sideband Handbook from 1958 is now out of copyright, as is Radio for the Millions. A lot of these old titles are now available from Lindsay Books.
As I've mentioned in other places, a lot of classic SF has expired, including most of E. E. Doc Smith's work, and much of H. Beam Piper. All of the Skylark books except for Skylark Duquesne (published shortly before the author's death in 1965 and thus outside the window) have expired, as have all of the Lensman books except for Gray Lensman and Children of the Lens. None of the Ace Double short novels I've checked have shown up for renewal, including Chandler's The Rim Gods and Lin Carter's Destination Saturn. Both of those could stand republishing; most of the other Ace Double entries I have are best forgotten. (It may be that the components of Ace Doubles were treated differently from a copyright standpoint; this would be useful to know. I'm looking into it.)
Nothing written solely by the Jesuit Herbert Thurston has been renewed, and his book Ghosts and Poltergeists is actually good sleepytime reading. (I'm still trying to obtain The Physical Phenomena of Mysticism, which of all his books has the best rep. The bookstores I order it from keep selling it to somebody else before I get there.) The New Dictionary of Thoughts is a decent book of quotations, well-organized by subject, and now expired. Max Freedom Long's pre-1964 books on Hawaiian religion and magic were not renewed, nor were Carl & Jerry author John T. Frye's two books on radio repair. Ditto Glenn's Theodicy and Broderick's Concise Catholic Dictionary, along with Jessie Pegis' A Practical Catholic Dictionary. The slightly peculiar Benziger Brothers' My Everyday Missal from 1948 (with print I can't imagine anyone could read in a badly lit church) does not appear in the renewal records. Ditto My Sunday Missal from Fr. Joseph Stedman (1942) and St. Joseph Sunday Missal from Catholic Book Publishing (1962). In fact, most of the odd little prayer books I've gathered over the years either have no copyright notice or were never renewed.
And that's just the stuff from my own library. When I come across a book published in the Weird Window, I often check the renewal records to see if it's expired. There's a nice lookup page here, though the lawyers always caution that it's possible for there to be errors. I suppose. Nonetheless, there's a lot of room for the release of these titles as ebooks, or their reissue in print via POD. The public domain does not begin in 1922 and go back from there.