Mass market paperback : Also known as the pocket paperback. A small format paperback usually measuring about 4" by 6".

spine : The outer portion of a book which covers the actual binding. The spine usually faces outward when a book is placed on a shelf. Also known as the back.

first thus : Indicates that this is not the first appearance of a book in print, but that this is the first appearance in a substantially different format than the true first edition. For example, a first illustrated edition of a book, or even a first paperback release of a book originally published in hard cover.