The New York Times ran an article the other day about how sales of mass market paperbacks are dropping sharply.
For years, as a teenager and young adult, virtually all the books I bought were mass market paperbacks, or cheap Science Fiction Book Club editions. Many of my favorite books were grubby little paperbacks. I still have my original ILLUMINATUS! mass market PB's from the 1970s.
Michael Johnson had a particularly good posting about book collecting the other day.