pocket book vs soft-cover

pocket book and soft-cover both are nouns.

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

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

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

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