paper-back book vs softback book

paper-back book and softback book both are nouns.

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

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

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

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