After almost a century in wide circulation, the mass market paperback is shuffling toward extinction. Sales have dropped for years, peeled away by e-books, digital audiobooks and even more expensive formats like hardcovers and trade paperbacks, the mass market’s larger and pricier cousin. Last year, ReaderLink — the country’s largest distributor of books to airport bookshops, pharmacies and big-box stores like Target and Walmart — announced that it would stop carrying mass markets altogether.