In convenient graph form:
Because of the strange distortions of copyright protection, there are twice as many newly published books available on Amazon from 1850 as there are from 1950 (via The Missing 20th Century: How Copyright Protection Makes Books Vanish)
This kind of thing is what infuriates me the most about capitalism. They aren't even following their own rules! You should want to sell more books and create more value, right? Right? But somehow hoarding IP and rent-seeking is seen as the better path to take. I just don't get it.
See also: ebook lending and libraries.