How long copyright terms make art disappear

Jill sez, "Exciting study samples new books for sale by Amazon and asks: Why are there three times more books initially published in the 1850's than books from the 1950's? The chart on page 15 is eye-popping, showing graphically decade-by-decade how many more new books initially published before 1923 are currently available than those published after 1923 [the magic public domain date]. The music and YouTube data are also compelling!"

How Copyright Makes Books and Music Disappear (and How Secondary Liability Rules Help Resurrect Old Songs)