A volume is a single book — i.e., a group of pages bound between two covers. Sometimes "volume" is used as a synonym for "work," but do remember a volume can contain more than one work (such as an anthology), and a work can stretch to more than one volume (such as an encyclopedia or a triple-decker).

From the Guide to Literary Terms by Jack Lynch.
Please send comments to Jack Lynch.
Note: This guide is still in the early stages of development.
Three question marks mean I have to write more on the subject. Bear with me.