pocket book vs soft-cover book

pocket book and soft-cover book both are nouns.

Word	Noun	Adjective	Verb	Adverb
pocket book	Yes	No	No	No
soft-cover book	Yes	No	No	No

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

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

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