A Mass Market Paperback is a type of paperback book intended for the mass market. Although the mass market paperback has been around in its current form for about a century now, and it has varied somewhat in size and form, the basic mass market paperback has kept the same format: about the size of a hand. from 100 to several hundred (and sometimes over a thousand) pages made of paper of medium quality, and sometimes with separate sections with better paper for photographs. If you think of a paperback, you are probably thinking of a mass market paperback.
Mass market paperbacks were published and printed in every genre, including classics and literature, but were often popular fiction, including genre fiction, as well as popular types of non-fiction, especially military history, true crime, and biography. Although paperbacks were made to be less durable, and many have doubtlessly been lost, there is still a tremendous backlog of mass market paperbacks floating around in the United States, in people's homes and attics, in used book stores, in thrift stores. Their price usually runs from free to cheap, and many a young curious reader, moving on from children's literature, first got a helping of adult books by finding old stores of mass market paperbacks.