soft-cover book vs paper-back book

soft-cover book and paper-back book both are nouns.

Word	Noun	Adjective	Verb	Adverb
soft-cover book	Yes	No	No	No
paper-back book	Yes	No	No	No

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

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

soft-cover book (noun)	paper-back book (noun)
a book with paper covers	a book with paper covers