When Congress passed the first Copyright Act in 1790, copyright protection lasted for 14 years, and authors could apply for a single 14-year extension. After a book's copyright term expired, the work would fall into the public domain and become available for anyone to reproduce it.

But Congress has repeatedly extended copyright terms. Today, new works are protected by copyright for the life of the author plus 70 years, and some works are still under copyright 90 years after they were published. Rebecca Rosen at the Atlantic spotted some research by Paul Heald at the University of Illinois law school that illustrates how the longer terms of modern copyright have affected the availability of older books. Heald examined a random sample of new (e.g. in-print) books on Amazon and plotted their distribution. His results were surprising:

(Paul J. Heald)

Why the dramatic drop-off in the number of titles being published after the 1920s? Works published before 1923 are known to be in the public domain, giving publishers the right to republish them without asking anyone's permission. In contrast, works published since 1923 may still be under copyright protection, so publishers have to do legal research to find the copyright owner (if any) and negotiate a license. That's a hassle, so works published after 1923 are much less likely to be republished once they've gone out of print.

There is an important caveats to Heald's results. The pre-1923 books include many duplicates — different publishers offering competing copies of the classics. Heald estimates that the average pre-1923 book has four copies available on Amazon. But Heald finds that even correcting for that factor doesn't change the basic picture. There are still a lot more titles in print from the 1890s, 1900s and 1910s than from the 1930s, 1940s and 1950s. And that's especially surprising because the number of books being published per year was going up during the 20th century. So, there are a lot of books from the mid-20th century that would be more widely available if copyright law wasn't standing stand in the way.

Congress last extended copyright terms in 1998, adding 20 years to most terms. That means that works from 1923 are scheduled to fall into the public domain again on Jan. 1, 2019, unless Congress grants yet another extension before then.