paper-back book vs softback

paper-back book and softback both are nouns.

Word	Noun	Adjective	Verb	Adverb
paper-back book	Yes	No	No	No
softback	Yes	No	No	No

As nouns, paper-back book and softback are synonyms defined as:

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

paper-back book (noun)	softback (noun)
a book with paper covers	a book with paper covers