Paul J. Heald used some software that crawled Amazon for a random selection of books. The preliminary results were startling:
There were as many books available in the 1910s as there were from the 2000s. The number of books from the 1850s was double the number available from the 1950s. Why? Copyright protections (which cover titles published in 1923 and after) squashed the market for books from the middle of the 20th century, keeping many titles off shelves and out of the hands of the reading public. Heald has now finalised his research and the results…. Read on…
"Copyright correlates significantly with the disappearance of works rather than with their availability," Heald writes. "Shortly after works are created and proprietised, they tend to disappear from public view only to reappear in significantly increased numbers when they fall into the public domain and lose their owners."