paperback book vs softback book

paperback book and softback book both are nouns.

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

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

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

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