softback book vs volume

softback book and volume both are nouns.

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

As nouns, volume is a hypernym of softback book; that is, volume is a word with a broader meaning than softback book:

Other hypernyms of softback book include book.

softback book (noun)	volume (noun)
a book with paper covers	physical objects consisting of a number of pages bound together
	the magnitude of sound (usually in a specified direction)
	the property of something that is great in magnitude
	a publication that is one of a set of several similar publications
	the amount of 3-dimensional space occupied by an object
	a relative amount