softback book vs paper-back book

softback book and paper-back book both are nouns.

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

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

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

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