I managed to sum it all up fairly short along with a brief description of why that is.
I didn't go into great detail on copyright policies because most of it wasn't really relevant to determining the Public Domain status.
Step 1: Copyrights by Date
Everything that was registered to the Copyright Office during or before 1922 is in the Public Domain.
The Copyright Act of 1909 granted newly registered Copyrights 28 years of protection. This also included one opportunity to renew the Copyright for another 28 years. This means any work can be Copyrighted for 56 years which means a work registered in 1922 and renewed in 1950 would fall in the Public Domain in 1978.
The Copyright Act of 1992 removed the need to register a renewal, so the Copyright was automatically renewed but only if a work had not already passed into the Public Domain.
Between 1923 and 1963
If a work was registered but never renewed, then it is in the Public Domain.
The Copyright Act of 1992 removed the need to register a renewal which granted all works an automatic renewal. However, if a work had entered the Public Domain prior to 1992 because it had not been renewed, then it was ineligible for an automatic renewal.
Therefore, if a work was registered in 1963 it was protected for 28 years but it needed to be renewed in 1991. If it was not renewed in 1991, then it was in the Public Domain and was not eligible for an automatic renewal.
Any work registered during or after 1964 will not enter the Public Domain for another 95 years (at the least).
The Copyright Act of 1976 altered the renewal from 28 years of protection to 47 years, and The Copyright Act of 1992 made renewals automatic (which included works that were not in the Domain yet). So a work from 1964 wouldn't enter the Domain until 2039. But the Copyright Term Extension Act of 1998 changed the Copyright period from 28 years to the duration of the Copyright holder's life.
So don't expect anything after 1964 to hit the Public Domain anytime soon. Unless of course it has never been registered.
Step 2: Is It Public Domain?
If it is registered between 1923 and 1963 but never renewed, then yes it is. But how do you know it was renewed or even registered to begin with?
The United States Library of Congress Copyright Office keeps records of all Copyright registrations.
You can walk in there (during office hours obviously) and peruse through all the records or you can file a request with the Copyright Office and they will peruse the records themselves for whatever it is you are looking for. But that means they will bill you for an hourly fee and other fees (I hear it gets expensive fast). Another method you can use is hire someone to do all this for you which would be cheaper. You can also try checking local libraries. The Library of Congress has printed and distributed Copyright catalogs so it is possible that your local library may have Copyright records. These catalogs contain records between 1891 and 1978.
But luckily, the Copyright Office now has an online catalog of records here:
This catalog includes records from 1950 and after. For some reason, the site says it only contains records from 1978 and after but it does contain records from at least 1950. However, they probably are not complete if you look for something before 1978.
But Stanford has its own catalog of Copyright Renewals available online here:
This catalog covers the Renewals from 1950 to 1992 but I believe it only covers books.
Step 3: Locate Copyright
On this book, the Copyright date is stated as 1935. So the registration would last until 1963 unless it was renewed.
Step 4: Search Copyright Catalogs
A quick search of the Copyright Office shows that this book has no renewal record in 1963. Sometimes, a renewal registration will appear a year earlier or after the expected Renewal deadline, depending on when the Copyright Office received the renewal.
In this case, there is no Renewal record in the Copyright Office's catalog so it might be in the Public Domain. Just to be safe, I searched the Stanford Renewal Catalog.
This catalog did find a renewal record from 1962. So this book's initial copyright from 1935 was extended in 1962, meaning it would have entered the Public Domain in 2010 but because of other Copyright Acts (which I don't want to get into) it will probably enter the Domain in 2030 (at the least).
Step 5: Renewal Rates
But before that, all Renewal registrations had to be done manually. If a work was copyrighted in 1923, that author would have had to submitted a Renewal in 1951 (28 years later). But that didn't always happen.
So in the case of the book I bought, the author had renewed it in 1962. But the Stanford Renewal Catalog shows that of the 22 books George Seldes published, at least 8 of them (before 1963) were not renewed. So those books that were not renewed are now in the Public Domain. He also published a 10 year newspaper that was never Renewed, so that is in the Domain too.
Step 6: Conclusion
So what can you do with Public Domain materials?
Well, you can reprint it, distribute and even sell it. No rights are reserved for the original author. Once an item enters the Public Domain, it is now public property.
I hope this guide helps you out a bit. If anyone knows of any further sites that offer more complete catalogs, send me a message or post them in the comments.