From Matthew Yglesias (3/30/12), one simple chart that illustrates why copyright terms are way, way, way too long for the good of the culture:
Books published before 1923 are in the public domain; we read a lot of them (based on Amazon shipping figures). Books published in the past 10 or 20 years or so are in copyright, but are still in high demand; they're making a lot of money for publishers and are encouraging a supply of new books.
Between these two periods, there's a vast desert of books that are still in copyright but are in very low demand–because the prices that copyright holders want to sell books for are too high for the market. (Publishers are presumably not selling older books at the prices that they would move at out of fear of hurting the market for new books that do sell well.)
If copyright lasted, say, 20 years, the way patents do, this graph suggests that there would be far more books read from the 1930s, '40s. '50s, '60s, '70s and '80s–at least 10 times as many, and probably far more than that, if you assume people today are more interested in the literature of the 1980s than they are in that of the 1910s.
The culture is allowing a huge number of books to go unread–just to allow publishers to make a relatively small amount of profit from the tiny minority of 89-to-21-year-old books that still get read. It's a great public policy–if you think Americans read too much.
UPDATE: This graph doesn't say exactly what I thought it did, though it makes pretty much the same point. It's not measuring volume of sales; if it were, the most recent decade would presumably be a lot higher. Rather, it's looking at a random sample of 2,500 fiction books (along with some books about fiction) available in the Amazon warehouse, and seeing when they were published. Public domain works from the late 19th/early 20th centuries are well-represented in the warehouse, whereas there are few from the copyright desert from 1924 up through the 1980s. You can hear Paul Heald, the law professor who compiled the data, explaining his research in the video on this page at the 12:44 mark.