pocket book vs paper-back book

pocket book and paper-back book both are nouns.

Word	Noun	Adjective	Verb	Adverb
pocket book	Yes	No	No	No
paper-back book	Yes	No	No	No

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

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

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