paper-back book vs soft-cover book

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

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

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

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

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