As you can see, there are a bunch of recent books, then a huge drop off... until a sudden spike at 1922 -- also known as the year before which nearly all books are in the public domain. That giant gaping hole on the right side of the graph should be pretty distressing. It counters the totally false narrative by certain legacy copyright system supporters that copyright is necessary to get books published and also that without copyright, no one would bother to sell the works, because they could just be copied by others. But, more importantly, it shows how much important culture is totally locked up because of copyright law -- unable to be published by those who'd like to offer them, and not worth it for the copyright holders to actually publish.
Late last year, EU Parliament Julia Reda published a similar chart concerning the EU:
That one also looked at books, in the same manner as Heald's original research. On top of that, Heald himself has continued to explore this issue, including comparing new books to used books and also looking at the music space.