soft-cover vs softback book

soft-cover and softback book both are nouns.

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

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

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

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