softback book vs soft-cover book

softback book and soft-cover book both are nouns.

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

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

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

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