pocket book vs paperback

pocket book and paperback both are nouns.

Word	Noun	Adjective	Verb	Adverb
pocket book	Yes	No	No	No
paperback	Yes	Yes	No	No

As nouns, paperback is a hypernym of pocket book; that is, paperback is a word with a broader meaning than pocket book:

Other hypernyms of pocket book include paper-back book, paperback book, soft-cover, soft-cover book, softback, softback book.

pocket book (noun)	paperback (noun)
pocket-sized paperback book	a book with paper covers

pocket book (adjective)	paperback (adjective)
	(of books) having a flexible binding