soft-cover book vs softback book

soft-cover book and softback book both are nouns.

Word	Noun	Adjective	Verb	Adverb
soft-cover book	Yes	No	No	No
softback book	Yes	No	No	No

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

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

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