paperback book vs softback

paperback book and softback both are nouns.

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

As nouns, paperback book and softback are synonyms defined as:

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

paperback book (noun)	softback (noun)
a book with paper covers	a book with paper covers