pocket book vs softback

pocket book and softback both are nouns.

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

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

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

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