It's sad. These were really affordable and covered escapist fiction (all kinds of sci-fi) but also a lot of serious and civic-minded literature.

This was pretty much inevitable. The real death-blow was the collapse of the independent distributors. The effect of centralized buying and distribution was fewer slots for 'midlist' books and more slots for lead titles, so for most books, it was harder to push out enough copies to actually make any money. Print runs declined and unit costs rose. It was easier to sell trade paperbacks in traditional bookstores, then ebooks came and ate the bottom of the market.

This will be the death of the used bookstore too and I very much worry for the libraries.