softback book vs soft-cover

softback book and soft-cover both are nouns.

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

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

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

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