paperback book vs soft-cover

paperback book and soft-cover both are nouns.

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

As nouns, paperback book and soft-cover are synonyms defined as:

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

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