softback vs pocket book

softback and pocket book both are nouns.

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

As nouns, pocket book is a hyponym of softback; that is, pocket book is a word with a more specific, narrower meaning than softback:

Other hyponyms of softback include pocket edition, pocketbook.

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