softback vs softback book

softback and softback book both are nouns.

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

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

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

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