But! A recent NYPL project has paid for the already-digitized registration records to be marked up as XML. (I was not involved, BTW, apart from saying “yes, this would work” four years ago.) Now for anything that’s unambiguously a “book”, we have a parseable record of its pre-1964 interactions with the Copyright Office: the initial registration and any potential renewal.
The two datasets are in different formats, but a little elbow grease will mesh them up. It turns out that eighty percent of 1924-1963 books never had their copyright renewed. More importantly, with a couple caveats about foreign publication and such, we now know which 80%.
Source: Secretly Public Domain
An update to his original post puts the number at 73% instead of 80%, but it’s still a lot of books!
Once a book is in the public domain, you can do whatever you want with the book or the characters of the book. Do you want to re-distribute? Go ahead! Write a play version? Sure! Use the characters in a sequel? Yeppers!
The only problem right now is getting a hold of one of these books that are now in the public domain. Project Gutenberg has some of the books available as ebooks, and as knowledge of this work grows, more books will be available.