softback book vs paperback book

softback book and paperback book both are nouns.

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

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

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

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