softback book vs softback

softback book and softback both are nouns.

Word	Noun	Adjective	Verb	Adverb
softback book	Yes	No	No	No
softback	Yes	No	No	No

As nouns, softback book and softback are synonyms defined as:

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

softback book (noun)	softback (noun)
a book with paper covers	a book with paper covers