This is not unusual as mass market paperback sales have been declined for the last twenty years. (whatever.scalzi.com)
Although there is a small decrease in paperback sales, the digital format has just seen a really huge increase in sales, leading people to believe that the traditional method is obsolete. (readersfavorite.com)
At some point I look for paperback sales to come back but I don't think it will come back to what it used to. (accrispin.blogspot.ru)